There's no fast way if you're measuring time by hours.
The fastest way to improve is just to play more in ways you find fun.
(In Osu!Mania, pain is bad, because that leads to injuries, which makes you play less.)
My recommendation is to make your own multiplayer lobbies with a star rating limit at the high end of what you can play.
In your case, you might want to make a lobby titled "4k Mania 4.5* - 5.5* Host Rotation"
Plus with Host Rotation (where every person passes host down to the next person after picking a map), you get introduced to a lot of new maps you might not have ever considered before.
But if you don't like multiplayer, then really all I can say is keep playing, and download lots of maps.