yes, 4* is really hard to reach. i don't remember how long it took me but i think also around a year.
my problem was: it felt like there was a wall between ~3.7* and ~4.1*. like the difficulty suddenly increased A LOT.
first of all i hope you have a skin you can play well with. i would recommend one that doesn't have a very bright kiai like the standard skin. to me personally it's way to distracting but that's up to you. if you're happy with it, stick with it and don't try to switch your setup too often unless you feel that's the fault (which it usually isn't)
the hard part in oni is that you have to learn how to do patterns with mixed colors. like ddk, kdd, ddkkd, kdddk or whatever. and that's a lot more variety. and somehow most maps just jump into it that's why in my opinion it's so hard to get there.
what helped me were a few maps that have those patterns but not too many of them. i'll try to remember some of the maps that helped me personally:
https://osu.ppy.sh/beatmapsets/110292#taiko/303142 (this is one of the easiest maps i know with mixed patterns. it's very good to start learning them in my opinion)
https://osu.ppy.sh/beatmapsets/713086#taiko/1508509 (same pattern difficulty but more stamina and speed)
https://osu.ppy.sh/beatmapsets/650733#taiko/1386494 (very hard for that level! it has LOTS of different patterns and a bit slower bpm which you should get used to a bit too.)
https://osu.ppy.sh/beatmapsets/533037#taiko/1247899 &
https://osu.ppy.sh/beatmapsets/482486#taiko/1033930 (these are probably the 'last proper step' i know before 4* which are not yet 4* but very close if you're really nearly there)
some of these are probably really hard for that level so if it's too hard try to go for a (good) pass at least maybe.
of course there were more maps and you should play a lot of different ones but these were, in retrospect, probably the ones that helped me most personally.
i know it's hard but keep going, i'm sure you'll make it. gl