diff --git a/CHANGELOG.md b/CHANGELOG.md
index 741623a..5cfe783 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,16 @@
 # Changelog
 
+## 0.15.4
+
+ARCH-1 step 4 (final). Fourth collaborator extracted from `TrayContext`. No user-visible changes. With this the audit's ARCH-1 incremental work is done.
+
+Changed:
+
+- **New `MountManager` class** (`src/MountManager.cs`) owns the rclone mount process lifecycle bits:
+  - `BuildMountArgs(profile, fullCache, rcCommonFlags, effectiveBandwidth)` — pure helper that assembles the full rclone `mount` argv with all the cache-mode, VFS, network-mode, RC, log, and bandwidth flags Pixelpipe has used since v0.5.x. Unit-testable without touching disk.
+  - `StartMountProcess(rclonePath, args, logJobWarn)` — `Process.Start` plus the v0.11.4 Job Object binding so the rclone child still dies with Pixelpipe.
+- **`TrayContext.Mount.cs` delegates** argv assembly and the spawn. The dialog flow (rclone-missing, WinFsp-missing, remote-not-configured, drive-in-use, post-launch result) stays in `TrayContext` because it's UI-coupled.
+
 ## 0.15.3
 
 ARCH-1 step 3. Third collaborator extracted from `TrayContext`. No user-visible changes.
diff --git a/src/MountManager.cs b/src/MountManager.cs
new file mode 100644
index 0000000..1d6acf3
--- /dev/null
+++ b/src/MountManager.cs
@@ -0,0 +1,69 @@
+using System;
+using System.Diagnostics;
+using System.IO;
+
+namespace Pixelpipe
+{
+    // ARCH-1 step 4 (v0.15.4, audit): fourth and final collaborator
+    // extracted from the TrayContext partial. Owns just the rclone mount
+    // process-lifecycle bits: argument assembly + Process.Start + Job Object
+    // binding. The dialog flow (rclone-missing, WinFsp-missing, remote-not-
+    // configured, drive-in-use, post-launch result) stays in TrayContext
+    // because it's heavily UI-coupled.
+    //
+    // This is intentionally a thin extraction. The audit calls ARCH-1
+    // incremental; later releases can pull more orchestration in once the
+    // shape of "what doesn't need a TrayContext" stabilises.
+    internal sealed class MountManager
+    {
+        // Pure helper, tests cover it. Builds the rclone mount argv that
+        // Pixelpipe has used since v0.5.x with all the cache-mode, VFS,
+        // network-mode, RC, and bandwidth flags in one place. Caller passes
+        // RcCommonFlags + effective bandwidth so this class doesn't need to
+        // know about the auth token / per-profile bandwidth resolution.
+        internal static string BuildMountArgs(RemoteProfile p, bool fullCache, string rcCommonFlags, string effectiveBandwidth)
+        {
+            if (p == null) return "";
+            string cacheMode = fullCache ? "full" : "writes";
+            string args = "mount " + TrayContext.QuoteArg(TrayContext.NormalizeRemoteName(p.Remote))
+                + " " + TrayContext.QuoteArg(TrayContext.NormalizeDriveLetter(p.DriveLetter))
+                + " --links"
+                + (String.Equals(p.MountMode, "network", StringComparison.OrdinalIgnoreCase) ? " --network-mode" : "")
+                + " --vfs-cache-mode " + cacheMode
+                + " --dir-cache-time 10m"
+                + " --poll-interval 1m"
+                + " --vfs-write-back 10s"
+                + " --vfs-cache-max-age 6h"
+                + " --vfs-cache-max-size 5G"
+                + " --volname " + TrayContext.QuoteArg(p.Label)
+                + " --rc " + (rcCommonFlags ?? "")
+                + " --log-level INFO"
+                + " --log-file " + TrayContext.QuoteArg(p.LogFile);
+            if (!String.IsNullOrEmpty(effectiveBandwidth)
+                && !String.Equals(effectiveBandwidth, "off", StringComparison.OrdinalIgnoreCase))
+            {
+                args += " --bwlimit " + effectiveBandwidth;
+            }
+            return args;
+        }
+
+        // Spawns rclone mount and binds the child to the kill-on-job-close
+        // Job Object so it dies with Pixelpipe even on Task Manager kill /
+        // crash / sign-out. Returns the started Process; caller is
+        // responsible for the post-launch monitor (1900 ms wait + exit
+        // check + UI feedback).
+        internal static Process StartMountProcess(string rclonePath, string args, Action<string> logJobWarn)
+        {
+            ProcessStartInfo psi = new ProcessStartInfo();
+            psi.FileName = rclonePath;
+            psi.Arguments = args;
+            psi.UseShellExecute = false;
+            psi.CreateNoWindow = true;
+            psi.WindowStyle = ProcessWindowStyle.Hidden;
+            psi.WorkingDirectory = Environment.GetFolderPath(Environment.SpecialFolder.UserProfile);
+            Process child = Process.Start(psi);
+            RcloneJob.TryAssign(child, logJobWarn);
+            return child;
+        }
+    }
+}
diff --git a/src/Pixelpipe.Mount.cs b/src/Pixelpipe.Mount.cs
index 3c1a147..c4938db 100644
--- a/src/Pixelpipe.Mount.cs
+++ b/src/Pixelpipe.Mount.cs
@@ -168,39 +168,16 @@ private void ContinueMountOnUiThread(RemoteProfile p, bool fullCache, bool rclon
             p.FullCache = fullCache;
             p.DesiredMounted = true;
             p.LastError = "";
-            string cacheMode = fullCache ? "full" : "writes";
-            string args = "mount " + QuoteArg(NormalizeRemoteName(p.Remote)) + " " + QuoteArg(NormalizeDriveLetter(p.DriveLetter)) +
-                          " --links" +
-                          (String.Equals(p.MountMode, "network", StringComparison.OrdinalIgnoreCase) ? " --network-mode" : "") +
-                          " --vfs-cache-mode " + cacheMode +
-                          " --dir-cache-time 10m" +
-                          " --poll-interval 1m" +
-                          " --vfs-write-back 10s" +
-                          " --vfs-cache-max-age 6h" +
-                          " --vfs-cache-max-size 5G" +
-                          " --volname " + QuoteArg(p.Label) +
-                          " --rc " + RcCommonFlags(p.RcPort) +
-                          " --log-level INFO" +
-                          " --log-file " + QuoteArg(p.LogFile);
-
+            // ARCH-1 step 4 (v0.15.4): argv assembly and process spawn
+            // delegated to MountManager. The post-launch monitor stays here
+            // because it does UI work (BeginUi, MessageBox, ShowBalloon).
             string effectiveBandwidth = EffectiveBandwidthFor(p);
-            if (!String.Equals(effectiveBandwidth, "off", StringComparison.OrdinalIgnoreCase)) args += " --bwlimit " + effectiveBandwidth;
+            string args = MountManager.BuildMountArgs(p, fullCache, RcCommonFlags(p.RcPort), effectiveBandwidth);
 
             try
             {
-                ProcessStartInfo psi = new ProcessStartInfo();
-                psi.FileName = rclonePath;
-                psi.Arguments = args;
-                psi.UseShellExecute = false;
-                psi.CreateNoWindow = true;
-                psi.WindowStyle = ProcessWindowStyle.Hidden;
-                psi.WorkingDirectory = Environment.GetFolderPath(Environment.SpecialFolder.UserProfile);
-                p.MountProcess = Process.Start(psi);
-                // Bind the new rclone to our kill-on-job-close Job Object so
-                // it dies with Pixelpipe even if Pixelpipe is killed via Task
-                // Manager / crashes / etc. Best-effort; the orphan-scan path
-                // catches anything that slips through.
-                RcloneJob.TryAssign(p.MountProcess, delegate(string warn) { LogUiWarn("rclone job assign " + p.Label, warn); });
+                p.MountProcess = MountManager.StartMountProcess(rclonePath, args,
+                    delegate(string warn) { LogUiWarn("rclone job assign " + p.Label, warn); });
                 p.StatusText = "mounting " + GetDriveRoot(p);
                 SaveProfiles();
                 RebuildMenu();