I have a bright drop in that needs 6 vdc. have it in a 3AAA host right now and want it brighter.

6V led typically means a linear driver such as an AMC7135 driver.

Without using CR123 or Li-Ion re-chargables you are spinning your wheels....unless you want large and bulky an inefficient.

4 AA is not large and bulky for my needs, and it can also be 4 AAA.  Anyone?

I mean seriously, 3 AAA is not bulky at all and it drives my red LED just fine.

This is a Solarforce P60 host.  It comes with (2) 18650 extenders.  You should be able to fit 4AA in the tube in series as long as you roll up some paper to take up the slack space.  You will have to figure out how many pcs of paper you need to get the AA to fit with no rattle.

http://www.solarforceflashlight-sales.com/product_detail.php?t=FB&s=1&id=157

Also, it's probably 6V max, not actually designed to run at 6V (Buck/Boost drive will burn off excess voltage as heat).  You won't necessarily get a brighter light from a higher voltage.  You may get much better results with 4AA Ni-Mh (Eneloop) batteries than you will with 4AA Alkaline Batteries.