The fifth alpha build for paint.net 4.0 is now available! This is the last planned alpha release, which means beta is next and it’ll become available to v3.5.11 users if they have "Also check for pre-release (beta) releases" enabled.
 
To install it, either use the built-in updater from a previous 4.0 alpha build, or head on over to the forum to download it directly: http://forums.getpaint.net/index.php?/topic/27804-paintnet-40-alpha-build-5143-is-now-available/

Here are the changes between the previous build (5105) and this one (5143):

• Cropping after several simple selection manipulations (e.g. move, scale) would often result in an extra row or column of blank pixels in the cropped image
• Move Layer Up/Down would often result in the wrong scroll location in the Layers window
• Panning the canvas via holding the spacebar and click-and-drag of the left mouse button was still panning even after the mouse button was released, but before the spacebar was
• Clone Stamp tool now uses the Primary color’s alpha value (or Secondary if you use the right mouse button)
• Fixed a crash that would happen if you clicked on the image list dropdown button (to the right of the image thumbnail list) too quickly after opening a large image
• The Move tools would revert changes made with the keyboard if the Enter or Esc keys were pressed too quickly afterwards
• Using the "Check Now" (for Updates) button in the Settings dialog did not close the application if you had any images open which had unsaved changes
• Fixed some issues with the Colors window whereby left clicking would reset the Primary/Secondary choice to Primary
• The floating window positions (Tools, History, Layers, Colors) were not being persisted across sessions
• Plugins can now retrieve the correct value for CurrentCulture and CurrentUICulture from any thread
• Fixed some rather pernicious crash bugs in several tools related to a "Committing changes" dialog popping up during click-and-drag operations
• Clicking and dragging on handles in the Gradient tool was lagging behind by 1 frame of mouse movement
• Fixed a focus bug on startup whereby you couldn’t use keyboard commands until you clicked on the canvas
• Fixed a crash on Win7 when switching from Aero to Classic theme
• Settings dialog now works correctly in High Contrast mode
• Fixed the pixel grid being off by 1 screen pixel (it didn’t line up with the ruler)
• Fixed a CPU usage issue with the Move and Shape tools. They were running the animations and UI code even if the UI wasn’t visible yet (e.g. no selection / no shape).
• The "OK" button wasn’t working in the New Image, Resize, and Canvas Size dialogs when the rest of the application’s CPU usage was very high (e.g. slow CPU, slow GPU, disabled hardware acceleration)
• Reduced CPU usage of animated UI elements, especially when not the active application or when the user has disconnected (locked workstation, Remote Desktop)

Enjoy!