It's neat that you added the turrets to the map, but I really think that a map with turrets needs to be designed specifically for the turrets in order to really work.
Before the turrets, the map was pretty decent. There are multiple entrances/exits for each base, but that is balanced by there being a good amount of weapons spread out and also the megahealth/armor in base to help the flag defenders, but also a megahealth/megaarmor in the side paths to help attackers. The big challenge about the map (for defense) is that there was a clear pathway between the bases and if the enemy flag carrier can get out of the base, then they will usually cap...that is, unless there is someone sniping in the middle room or their own flag is not in their base. Getting in and out of the base with the enemy flag seems to be the challenge for attackers.
In Gforce3, you opened up more openings for the base (the ones that were in Gforce1), which I think is good because it makes defense a little bit harder. However, you took out the megahealth and /megaarmor from the base to put the turrets in. The lack of health/armor in base definitely makes defense harder (those turrets don't make up for it)
My suggestion for Gforce4
I suggest removing the turrets and put those on another map that is designed specifically for turrets

and place the MG and MH back in base and plan a Gforce4 that expands a bit more on what GForce3 has to offer. I could imagine the map expanding a bit in size....perhaps make it a little bit longer so there is a larger middle area. Then put another obstacle in the middle so that pathway isn't so clear. Perhaps make a second level either above or below the middle area, maybe some pits in the ground to slow things down, .or maybe some floating platforms with items on them. I dunno...just thinking out loud here. Just something to make the middle area a little more dangerous of a path would be good. Length of the map would be good (similar to lavaflag...but perhaps not that long). You could even increase the length of the bases so there is more room for players to move around. Perhaps you could put a couple floating platforms in base to make things interesting. That could actually add more of a challenge to the defense because attackers could come in and jump up to the platforms to help them evade, and maybe grab some health/armor/weapons before grabbing the flag.