I assume you've thought of a weightbelt and rejected it for some reason.
If you only need 4kg and you want it ditchable, how about the little weight pockets Halcyon make for slipping on your tank cam band (for trim)? They take 2kg each.
http://www.dirdirect.com/astro/Trim_...Pocket3958.htm 
The weights are held in the pocket by a velcro flap and a little QR buckle. You could mount them upside down on your waist strap. The buckle would prevent accidental dropping of weights; when you wanted to ditch them just unclip the buckle and pull the velcro flap open. If you use shot bags instead of blocks, they should fall out easily.
Might be a bit of a bodged solution - and I'm sure there's a better one - but this would be about half the price of the ACB thingy.