I want my carbine simple as posilbe, so i'd prefer not to have to change out my handguards just to add a foward pistol grip and flashlight.  I haven't been able to find much about the stability of the attachable rails however, and would like to know if there are any good ones out there that are worth considering. or should i just get the new hand guards and put covers over all the unused rails (extra $$$).

I was like you I didnt want a 4 rail handguard, thought it was unnecessary. I put a set of rails on the bottom of my M4 HGs, and a VFG. I then attached a short rail section o the 2:00 portion of the HGs, and attached a light.

The factory plastic is nowhere near strong enough for the VFG. It will move. A lot. A whole lot. So much so, that I feared I would break them, or twek the gas tube.

I have one of the add on rails made by MI. The rail itself is plenty sturdy and would be fine for just a light. The problem is the plastic handguards just aren't tough enough to support real use of a VFG. It will last if you just plink at the range with it, but if you intend to really use/abuse one you're best served to bite the bullet and go with a real railed handguard. The two piece MI and YHM can be found here on the EE for $99-$129 new.

Check out GG&G. they have the rails that attach to the regular M4 guards.