I staied at the MO in Chiang Mai at the beginning of September.
I have to say its the best hotel i have ever staied in. The service is top notch, the rooms are spacious and beautiful, the infinite pool is out of this world. The surrounding make you feel like you are in a thai rice village. The hotel is unbelievble.
Don't worry about the "teething" issue, there are many hotels out there that can only dream to have service like at this MO. (Including the MO in HKG).
Don't worry, I travel alot and have staied in many high class resorts (this year I have already been at 4-S in Costa Rica, Wailea, MO - HKG, London, Park Hyatt in Jo'burg).
I haven't been in the Four Seasons in Chiang Mai, but i can safetly say that you can't go wrong with as long as a stay at the MO.
BTW - One final note, after i check out eventhough I took a taxi to the airport ( as opposed to the hotel's transfer) they had someone from concierge waiting for me at the airport to assist me with checking in. She even had them hold me a bulk head seat before I arrived at the airport. AMAZING!!!!!

I've also stay at the MO Chiang Mai twice and I can confirm that you'll love it. The promotion rate has been in place since the opening of the hotel and the FS has counter with an even lower rate.

You tourist always talk about mosquito repellents but you never talk about what to do after being bitten. There's something you can buy locally which my whole family uses and it's call "sambuck" a green colored balm that's very soothing.

We were in CM 2 weeks ago and many of the hotels are still effected by the floods. One of the new luxury hotels--Mandarin Chedi was closed at that time and did not expect to be open for 3-4 months. Several people got to their hotels and found the rooms unacceptable and moved. The hotels by the river still had some flooding. One popular guesthouse there, the customers had to wade in calf deep water in the lobby. I suggest checking directly with the hotel if you are going in the next few weeks.

Stitchintime-- The Chedi and the Mandarin are two different hotels. I know the Chedi was affected. Have not heard anything about the Mandarin.

Do you remember the name of the guest house that was affected?

No, I cannot remember which one it was. I do know that several places on both sides of the river were affected. The photos I saw in CM, the river banks were well up into the tree limbs on the trees by the river.