Keep in mind the resort is a good distance from Chiang Rai.... 3 nights is really perfect here. If you must have 4 and the resort is at full occupancy it will be hard for you to get in with additional elephant time. Perhaps an additional spa treatment. To be honest with you, I found the long tail boat ride and the city visit very lame. I skipped the Opiuim centre in favour of some pool time.
We did four nights here in August and are really glad we splurged on the extra night. We also skipped the Opium centre and the city visit because they didn't really grab us, and we skipped the Burma excursion for political reasons. We heard from others who went on these outings that we made the right call. We did spring for the cooking class and really enjoyed that immensely. Spa experience was sub-par but I get the sense that we just had bad luck and others have had better experiences.
For what it's worth, I recommend going for four nights and doing just the activities you enjoy and at the pace you enjoy. If you go for 3 nights and do all the activities they plan for you, you barely have any downtime to just relax and enjoy yourself.
Highlights were the elephants, lounging on our terrace, meals which were consistently excellent, the cooking class, sundowners at Burma Bar, and that bathtub.
Downsides were the pool and the spa experience. Really absolutely worth the trip but having done it once we're more likely to keep trying different places now.
Like the OP, we came back from the Tented Camp wanting to find other properties that delivered luxury but still broke loose a bit from the standard resort idea. So we'll be spending this new year's eve at Aman-i-khas, combined with a few nights each at Amanbagh, Amarvillas, and - we hope - the new Aman in New Delhi if it's open.