For such a small empire I would prefer Tradition. I would say if I want to create cities more than 6-7 then I'll choose liberty.Number of cities at 4?
As MadDjinn said: 1.
Liberty is a great policy tree in general, the game designers evidently tried to make it specialized towards expansion but they managed to just make it great. Under current circumstances there is no reason I would forgo liberty.
1.
Taking Liberty to the end gives a free Great Person of your choice. So 1 is the minimum number of cities to justify it. (not to mention the free Settler to give you a 2nd city, free worker, free golden age, extra production and meritocracy)
Number of cities at 4?
I usually end up taking the Tradition opener (to get to early policies faster)
I'm finding I almost always take the Liberty opener and then go for the settler and worker. They are just such a huge boost early on.
So I think any size empire can benefit.
Are you playing on duel map? Why would you want to have one city or even just four cities?