I currently run a 5mm trinity bullet for my shorty lipo packs, but am extremely limited on battery choices with 5mm. I am thinking about going down to 4mm, but use a 5mm adapter. Does anyone know where I can pick some of these up?

Pro-Match sells these:
http://www.promatchracing.com/proddetail.php?prod=6005
Whoops, you would need them the other way around for what you would want to do.

TQ racing has these and are awesome. 4mm/5mm step-up bullets. Use any 4mm or 5mm battery with one connector.

http://www.tqracing.com/5mm_bullets.htm

Thanks guys, I appreciate the quick responses!