All foregrips in the game are described as:
Aids in the manuipulation of the firearm
You can pick up the V4 in central Monte Puncu province in a cartel base. The accessory case is inside the armoury on the west side of the base.
I’m afraid I approached this completely unprepared, so this guide doesn’t provide the same level of information as some of my other quick tips. But this base had a lot of enemies, and I riled them right up and they called in a helicopter for reinforcement. This makes it difficult for me to recommend a good approach for this accessory, but even though I was being attacked hard, I lingered around the entrance to the base, killing any and all enemies who came out to greet me.
You may want to try approaching via the jungle to the west of the base for a more direct route to the armoury if you want to collect this accessory with the least fuss. And I’d recommend trying this at night.
The foregrip attachments (from the ‘Vertical Foregrip’ to this V4) all provide the same benefit and trade-off, just to varying degrees. Each improves handling while sacrificing a little accuracy. The V4 provides the biggest improvement to handling over all the other foregrips available, but with the most reduction to accuracy.
Having said that, there are no downsides to equipping this on a sniper rifle that accepts underbarrel attachments, such as the G28 (available in San Mateo). It would be one of my top recommendations, but for the fact that not all sniper rifles can accept it.
Personally, I like to rock a foregrip on an LMG or shotgun for the added mobility.