Since when does a 20MHz clock make a 2uS period?

If you're running at 20MHz... then why not try...

TRISB.4=0
Loop:
Toggle PortB.4
PauseUS 4
Goto Loop

PauseUS doesn't go down to 4uS at 4MHz but it just might work at 20MHz... at 4MHz you could try...

TRISB.4=0
Loop:
Toggle PortB.4
@ NOP
@ NOP
@ NOP
@ NOP
Goto Loop

Since each @ NOP will eat 1uS.