4mm is the standard size... the 3.5mm ones are normaly used with radio controlled models, between the brushless motor and the speed control.
If your chasing 3.5mm banana's, try a decent hobby shop.
Eric, I honestly wouldn't use the PP0400 and PP0402's, they are rubbish, I try my hardest not to sell them. The piggyback ones are much better, tho $1.95 each they will last forever. (PP0390, PP0391)
The PP2007 socket is okay, as long as your not intending to draw much more than a few amps.
Considering the aligator clips on the PP2007 aren't needed, get the PP2003 socket(same as used on the 2007) for $3.95 and some WB1706 twinflex speaker wire($0.80/M... OK for a couple of amps) and the piggyback banana plugs and it's all done, and cheaper
Edit: For higher current handling I'd recommend the PP2002 socket.