Just received a G4-A1 DD RIS model. Performance is impressive right out of the box - 420 FPS & 17 BPS.
I swapped the inner barrel with a Madbull V2 6.03 tightbore. Also swapped the stock hop up for an Element airseal unit and installed a harder bucking. These mods raised velocity to 430 FPS (this is a plinking gun so no worries about field limits). Accuracy improved a bit as well.
I added a Magpul sling and front sling adapter, a KAC vertical grip, ladder rail covers, a Madbull Gemtech mock suppres0r, and a Primary Arms Micro Dot on a QD riser. (I have an AAC/Blackout flash hider on the M4 barrel extension - haven't decided which I like best at this point)
I have one more mod planned - I'll be swapping the upper/lower receiver with a G&G Top Tech metal receiver set.
Here it is (crappy cell phone pic - sorry):