One of the projects pending is going to be the XM4, but I'm just looking to research.  From what I've seen, the XM4 had 3 different barrel configurations, ranging from a 14.5 inch pencil contour to the "traditional" M4 barrel.  Additionally, it seems that they had the 4 position stock, with initially the aluminum and then the fiberlite stocks.  It was initially set to be the Safe-Semi-Burst, but as I recall some flipped them to the Safe-Semi-Auto lowers that were in inventory due to the 4rd burst issues, where it would create the extra firing.  

My thought was to do the 6 hole carbine handguards or the "A2 cut-downs" that were talked about in Black Rifle II, but I'm uncertain on whether they were actually applied.  Aluminum stock with 4 position buffer.  The book talks about a heavy weight buffer, but doesn't state whether this became the new normal buffer, as one of the weights was replaced with tungsten...but I haven't done a weight comparison, as I don't have a full round buffer in my possession.

Thanks in advance, and as always, I'll try to keep people in the loop with regards to these projects.
v/r,
Jon

Going w/ the fixed rear sight/carry handle?

It will be the fixed A2 upper.  I recently grabbed one of the two XM4 receivers that popped on the EE (I'm a sucker for unique/rarer components)....so it'll be the A2 style upper with the M4 feed ramps anodized.  
v/r,
Jon

http://www.ar15.com/forums/t_3_123/688827_early_xm4_barrel.html

What is the rifle exactly that you're trying to replicate?  

The XM4 was an experimental variant, so it went through a lot of changes from its original inception to the finalized variant.  Are you trying to build a replica of one of the very early prototypes?  Just something that's generally representative?  

The "Heavy buffer" is the standard "H" buffer.  Adding a second tungsten weight created the "HH" or "H2" buffer, etc.

~Augee

Correct me if I'm wrong, but wouldn't fiberite stocks be the correct choice? I thought the earliest stocks were aluminum, then vinyl acetate coated, and later fiberite. It seems logically the aluminum would have been phased out by then.

But then again, I know Colt's transitional period is extremely varied and difficult to pin down.

The XM4 program started a good bit earlier in the 1980s than most people realize--it originally started as at the Army's request (9th ID, IIRC), then picked up by the Marine Corps for a while, both times as an SMG replacement (think M3 Grease gun more than M3 Carbine), before being finalized as the M4 Carbine.  

To that end, it wouldn't surprise me if some of the very early prototypes were built with leftover aluminum stocks, but I suspect they probably would have been two position as well, I can't say specifically what order the XM4's improvements were developed, but I suspect adding stock positions was a far lesser priority than getting other kinks worked out, and may not have really even been considered until after M16A2 carbines had been in use with various SOF units for some time and many of them began having their armorers drill extra holes in their receiver extensions.  

Reed Knight has an XM4 prototype in his collection which at very least the presence of an aluminum, vinyl acetate coated stock can't be ruled out:



(5th from the bottom, facing left, with an unproduced experimental barrel profile similar to that of the Sig M400)


Based on the detail visible, and comparing the "sheen" to the 639 above it and the 653 below it, I would say that it's most likely a vinyl acetate coated stock rather than a Fibrite.  

With developmental prototypes manufacturers will often build a prototype essentially to test one or another feature(s), and then just use whatever parts are available and close at hand to finish the prototype, and prototype configurations may not accurately reflect what the final configuration is intended to be.  

~Augee