Good, but missing a few things...
I liked it, I played it all the way through, but I cam out of it not feeling like I had acomplished something. Basically, no bosses.
In terms of level progression, all it is, is a list of levels, and you have to pick a level number. There wasn't a sense of "im getting there" it was very samey, with only adding in another enemy type every now and then, and only 4 backgrounds.
THe guns, were quite disapointing. I got the "big" gun, and didn't need anything else. The missile launcher wasn't as good, and the plasma cannon was a joke. Complete waste, of a helluva lot of money.
The graphics served their purpose, but you can do alot better. Flashier explosions, maybe a particle system would work good.
The general speed of the game was very slow, the more health the enemies, it seemed, the slower they moved. This was very annopying on the big map. Not so much the smaller ones.
I know it seems like alot is wrong with the game, but given all of that, I still played and completed it. If it was that bad, I would have stopped playing ages ago.
Tips for Metal Arena 4:
Try making the campaign more progressive, maybe add a map or something, so your fighting to take provinces, nothing massive, just a graphic over view of your progress.
Why not have different classes of tank?
-Quick, agile, not very powerful cannon and low health
-Medium, well medium everything.
-Slow, heavy, medium cannon and great health
It's a popular system, but there's a reason for that; it works.
That's all I can come up with for now.
Good luck mate, I enjoyed this game, hopefully you'll make another one that will be even better!