Death Blocks 4 is a decent FPS, with it not being nearly as grindy as the earlier games. The zombies are somewhat varied, the guns feel alright, and I haven't really tested multiplayer that much because no one is really playing. However, there should be easier customization of the buttons, there is a multitouch glitch where if you're playing with two fingers for aiming and firing, you can't fire if 1 finger is already on the screen. There are some frame rate drops whenever explosions happen, and firing a mech's weapons also cause occasional frame drops. Lastly having more guns would be nice and better gun customization like weapon skins or the option to paint your gun. That and there needs to be an option to buy gun optics with gems, as getting 1000 kills just for an optic would take forever.
But overall, it's a decent FPS that even has a map creator so you can build your own maps. It just needs more players.