[Bugs galore in the 4.0 update for me!]

I have the same issue. Disabling Hardware Accelerated Rendering (GPU) works for me too.

 

Application	paint.net v4.0 (Final 4.0.5288.36565)
Build Date	24 czerwca 2014
	
Renderowanie przyspieszane sprzętowo (GPU)	False
Animacje	True
Język	pl-PL
	
OS	Windows 8.1 (6.3.9600.0)
.NET Runtime	4.0.30319.34014
Physical Memory	3 523 MB
	
CPU	AMD A6-5200 APU with Radeon(TM) HD Graphics    
    Architecture	x64 (64-bit)
    Process Mode	64-bit
    Speed	~1996 MHz
    Cores / Threads	4
    Features	DEP, SSE, SSE2, SSE3, SSSE3, SSE4_1, SSE4_2, XSAVE
	
Video Card	AMD Radeon HD 8400
    Dedicated Video RAM	1 513 MB
    Dedicated System RAM	0 MB
    Shared System RAM	1 536 MB
    Vendor ID	0x1002
    Device ID	0x9830
    Subsystem ID	0xFA341179
    Revision	0
    LUID	0x0000C054
    Flags	None
    Outputs	1
	
Video Card	Microsoft Basic Render Driver
    Dedicated Video RAM	0 MB
    Dedicated System RAM	0 MB
    Shared System RAM	256 MB
    Vendor ID	0x1414
    Device ID	0x008C
    Subsystem ID	0x00000000
    Revision	0
    LUID	0x0000C60F
    Flags	Software
    Outputs	0