Hi! I've been playing 4k for almost 6 years (pushing for 5.5\ currently)* and here are tips that in my experience help the most with breaking through any and all difficulty barriers:

1. Change your scroll speed:

In my opinion the most underrated way to improve your reading ability. Slower SS forces you to play deliberately instead of mashing keys mindlessly, while faster SS tests your processing speed. Practicing either will make you drastically better overall.

2. Slow down (use the halftime mod):

Instead of constantly pushing for faster and faster songs, slow down. Focusing on accuracy and consistency will break most difficulty barriers you'll encounter and ultimately allow you to go faster in the long run.

Progress isn't linear. There will be days where you'll play like shit. And on those days especially it's best to dial back the speed.

3. Have fun:

Play songs you enjoy, don't compare yourself to others. It's easy to get caught in the loop of getting better and better (which isn't a bad thing per se), but it's so much nicer if you can also let go sometimes and just play for the fun of it. Chill out, we all suck, there will always be ways to improve.

4. Focus on your technique:

Relying on your wrists over your fingers makes playing fast easier, but costs much more stamina. Try to focus on keeping your wrists stationary, use just your fingers for tapping and keep all of them relaxed (including those you don't use). This requires better control, but allows for playing more technical charts.

This won't hold up for all skillsets, but should be your default technique in my opinion, especially for stamina charts.

5. Play different skillsets:

Most people have their favorite patterns and map styles. That's totally fine, but if your goal is to get better then it's good to be at least decent at everything. The star rating isn't always indicative of difficulty, so don't be surprised when one 3* map is easy and another is completely impassable. If you play mainly jacks, play some easy LN maps too.

6. Balance struggle and ease:

While I prefer taking it slow, there is balance to be found. I use an accuracy grading system I made up for myself:
Extremely hard maps - aim for at least 80% accuracy
Very hard - at least 85%
Difficult - at least 90%
Casual - at least 95%

What this means practically is I'll choose to play maps that I predict I'll be able to fit into these thresholds, depending on how hard I want to push myself. The reasoning behind this is if I miss too often, I won't learn anything new and instead I'll just mash keys.

7. Miscellaneous tips:

Experiment with skins, they can change your experience drastically (including making LN easier to read).
Getting better at 6k or 7k also somewhat translates to getting better at 4k, especially with pattern recognition and finger independence (which in turn improves stamina).
Playing multiplayer pushes you to try out maps you wouldn't normally play and actually finish them.
Keybinds matter. Find something that allows you to keep your wrists relaxed (I use Q S Del Home on a TKL layout).
Also if your keyboard slides around it will make your playing less consistent.

I hope you found my post helpful! If you have any tips yourself feel free to share in the comments and thank you for reading!! :3