I will not give an opinion if you still can learn as a 4 dan in go or not, because I don't know
I could still learn very much in go. 9 dan is the highest rank and then there are title matches between the strongest 9 dans. That is out of reach for me in my lifetime. But 5 dan would be a realistic goal.
The thing is that I feel that I
stopped learning years ago, even though I spend time on go. I could add 10 more years without improving at all. I know I should increase the intensity of my training to improve, but my motivation for that is lacking. It would come down to spending a lot of effort for a small improvement (or perhaps no improvement at all). That is one of the reasons I was looking for a new challenge, which I found in aikido. It feels so good to get noticeable improvement from effort spent.
Can aikidoka recognize this feeling? Have you experienced long episodes where you lost motivation because you stopped improving?