I stuck a G&P PEQ 9.6v 3300mah on my CA36 and it did add a fair bit of weight but thats because the rails are located almost at the very front of the weapon. On an M4, I'd figure it would be more balanced as the rails where you'd most likely mount the battery is a little further back.
"Hey I'm the first one to say its a great country but its a straaaaange culture. This is a place where gun store owners are given a list of stolen credit cards but not a list of CRIMINALS and MANIACS. And now they're thinking of banning toy guns...AND THEY'RE GONNA KEEP THE F**KING REAL ONES!"
-George Carlin 1937 - 2008 (RIP buddy)