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