I think I actually like this more than 4.1. I skipped the story completely, so I can't comment on its quality. I did notice it is less campy, but I'm indifferent to that.
Things I like:

• Weapons aren't locked to classes. Makes it more free form to choose the movement/weapon combo you like.
• Caverns are easier to navigate. 4.1 had me getting lost with its dark, labyrinthine caverns, but Iron Rain streamlines them a lot.
• More movement options. In 4.1, Wing diver felt like the only class that wasn't miserable solely because other classes were so slow. Iron rain adds dashes and PAs, making movement feel better across the board.
• I think the armor system is improved. Instead of grinding armor like in 4.1, upgrading armor takes currency, which you will get by naturallly playing the game anyway.

Things I don't like:

• I literally do not understand how weapon drops work. I hated 4.1's RNG, but I literally do not know how Iron Rain's works at all.