This definitely feels like it took inspiration from Ultrakill, given the general flow and style of both the UI and the visuals - and that works out pretty well here. The gameplay is a bit on the rough side, but given everything was made solo for this, you did really well for four days!
I think improvement wise, some feedback on the weapon firing would go a long way, and maybe some enemy variation? Or even a bit of music to fill the empty and drive the fight. Ultrakill and such works best when you have numerous enemy types to juggle, and have to make those strategic choices and decisions in order to get through a fight by picking your battles.
Pretty decent for four days' work! Nice job especially with everything being made solo.