Is a log cabin thing in the garden a good idea for a 15 year old? I know this is not the question. But that gives them a lot of independence to get up to all sorts at quite a young age. Would be they feel a bit excluded, woukd they get scared (I would have done, I wouldn't like to be cut off from the family like that).
I would only do a log cabin as a bedroom, say if I had an 18+ year old who was away at uni most of the time.
If 4 bedrooms is your budget, I would be looking for one with a downstairs room you could make into another bedroom.
Can you look at houses where you can do a loft conversion / ground floor extension?
To answer your OP, 12 year old gets smallest room. It's age order. When 20 month gets own room they would then get the smallest room.
The only other option would be for the 12 year old to get the biggest room then share with the 9 year old (if same sex).
Or for the 9 year old to share with the toddler in the biggest room.
But I would look for a scenario where all kids could have their own room in the main house even if it means doing an extension, using a downstairs room or splitting the biggest bedroom in two.