Hey there Chris, when you say bunny hop, do you mean a sidehop? 
Because to bunnyhop up a 4ft wall whilst facing it seems like a pretty impossible trick, with there being no run up an' all.  
 
The only thing I can say about pedal ups though, (or straight to backs), is that the hardest you push yourself, the higher you'll go. 
If you just start at say 2ft, then just go 2 or 3 inches higher each time, have a couple of goes at each height to make sure you got it, then move on to a wall just a tad higher. 
If you work like that then you'll also figure out pretty soon your own boundaries, and also your own courage.