S.E.
I'm an HISD teacher. Free HISD pre-k is only available for children who meet certain criteria such as: homeless (which includes children in foster care), ESL, etc. There are tuition-based pre-k programs (I believe Garden Oaks has one...last time I checked it was $440/mo.).
Some charter schools offer free PK3/PK4. My son attended Houston Heights Learning Academy (in the Heights) when he was 3. It was completely free (and we didn't have to meet any 'special' criteria). I believe the Varnett School has PK3.
In what area do you live?