[Bugs galore in the 4.0 update for me!]

I had the same problem. Turned off GPU rendering and it works now.

 

Diagnostics:

 

Application paint.net 4.0 (Final 4.0.5288.36565)
Build Date Tuesday, June 24, 2014


Hardware accelerated rendering (GPU) False
Animations True
Language en-US


OS Windows 7 Service Pack 1 (6.1.7601.65536)
.NET Runtime 4.0.30319.18444
Physical Memory 8,104 MB


CPU Intel(R) Core(TM) i5-2430M CPU @ 2.40GHz
    Architecture x64 (64-bit)
    Process Mode 64-bit
    Speed ~2394 MHz
    Cores / Threads 4
    Features DEP, SSE, SSE2, SSE3, SSSE3, SSE4_1, SSE4_2, XSAVE


Video Card NVIDIA GeForce GT 520M     
    Dedicated Video RAM 978 MB
    Dedicated System RAM 0 MB
    Shared System RAM 3,071 MB
    Vendor ID 0x10DE
    Device ID 0x1050
    Subsystem ID 0x16F21043
    Revision 161
    LUID 0x00008A17
    Flags None
    Outputs 1


Video Card NVIDIA GeForce GT 520M     
    Dedicated Video RAM 978 MB
    Dedicated System RAM 0 MB
    Shared System RAM 3,071 MB
    Vendor ID 0x10DE
    Device ID 0x1050
    Subsystem ID 0x16F21043
    Revision 161
    LUID 0x000092DF
    Flags None
    Outputs 0