Robert, I believe levels 1 through 3 are faster due to years of complaints from users saying Wanikani starting slow. The SRS time for those are half the time compared to the rest of the levels. Level 4 and above, the SRS levels are in the normal time range as specified in the Wanikani user guide. That’s probably why you’re finding level 4 slower than previous levels.
I love this graph. It should always be the first reply to threads like these. Although, I do understand the feeling of excitement when starting studying and wanting more.