The game looks great.
It's all little things I believe need to be fixed:
Enemy missiles are too hard to spot, perhaps a brighter color.
Enemy ships should be louder when they fly down towards your ship.
Give unique enemy ships different patterns for when they fly down.
Make some enemies faster. (And maybe give one of them a decent homing missile.
More upgrades. (fire rate, damage, HP, hull capacity (increase power-up holding capacity) maybe give each player available ship 2-3 skill trees. Each tree being a different type of weapon, and each basic weapon have 4 different "fully upgraded" versions at the end of its tree.
All of that... and damn that would be the SHIT! Lol.
But still a good game. Solid 4.