comparison-anaphor

Are all of these sentence correct?

1."The streets in LA are dirtier than in Stratford."

2."The streets in LA are dirtier than the streets in Stratford."
3."The streets in LA are dirtier than the ones in Stratford."
4."The streets in LA are dirtier than they are in Stratford."

When making comparisons, is an anaphor always needed after "than" or can I omit it as in the 1st sentence?

They all look OK to me. 1. is the simplest; 2. and 3. are perhaps clearest.

They all look OK to me. 1. is the simplest; 2. and 3. are perhaps clearest.