DivinityGate, you could learn to 7k if you really wanted. Spend one week playing only 7k for your entire osu!mania session and see how quickly you improve.
7k actually isn't a lot harder to learn than 4k is when you start out, contrary to popular belief. A very good part of why 7k feels harder is because most people play 4k first. By playing 4k first you only learn to handle 4 columns. Trying to learn 7k after 4k becomes much harder because now you've trained your brain to play 4k and you're trying to use your 4k skills to play 7k, which takes your brain a while to figure out how to adapt. On top of that, since you're aware of how much worse your 7k is than your 4k, 7k feels less fun because you are used to seeing better scores.
You can't compare mapping 7k to mapping 4k. The things you need to pay attention to in your patterns are completely different. The base theory of mapping remains the same (layering is important in both 4k and 7k, etc.) but the kinds of patterns and issues that involve those patterns are completely different. I don't think either is harder than the other when it comes down to creating truly good maps. I think people generally don't understand just what the difference between a 'good' and 'bad' maps is.
IN2k6's 7k lunatic map is absolutely terrible. To someone who can't play it, it doesn't look nearly as bad as it actually is. The patterns are much harder than they look, and it just flat out feel bad to play.