First thing: If you seriously want to learn 7k, I suggest you focus on ONLY playing 7k for a while, until you reach the skill level where you can hit patterns without having to think about anything. Any time you spend playing 4k or other key modes while you try to learn 7k will make things significantly harder.
Second: I would suggest practicing 7k until that skill level before trying 6k or 5k, for example. Finger independence and the ability to read patterns in lower keymodes than the highest one you've learned comes much more naturally than when you're trying to learn a higher keymode.
Starting out will be rough, and it will take longer than it did to learn 4k. However, once you've learned to play 7k, it's not hard to go back to any key mode lower. It may take some time to build your 4k skills back up, but don't worry, they WILL come back.