You're on schedule with the nitrite; my concern is that the ammonia is no longer dropping. Usually ammonia drops to 0 first, then the nitrite spike happens. Because your ammonia has slowed my thought was that the sand was playing a role. I could be wrong, but that's my best guess. The nitrite spike usually takes up to 3 weeks so you may just need a bit more patience and see if things can progress on their own. It's up to you, as I mentioned before you could drain the tank and stock slowly and try a fish-in cycle, but with only 4 weeks into a fishless cycle it's hard to say for sure whether there's an issue with the sand, I only mentioned it b/c as I mentioned the ammonia conversion has slowed. Personally I'd wait it out a couple more weeks and see if anything changes.