Kagehitokiri is right: Four Seasons-- 10 minute walk at most, allowing for very crowded sidewalks.

Then it's...
Conrad: 20 minute walk or so. Cab do-able.

Banyan Tree: 10-15 minute walk to Sala Daeng BTS Station, one stop on SkyTrain, then you're almost at the Four Seasons. Or take the SkyTrain one more stop, change at Siam, backtrack kind of to ChitLom and walk from there about 10 minutes.

Metropolitan: Basically same as Banyan Tree. They're practically next door to one another on Sathorn Road. A cab would be much better from either the Met or the Banyan Tree. Depending on traffic, could be around 10 minutes by cab to Central World Plaza.