I understand why you are worrying.... but don't. There are several reasons why his behaviour may have got worse.... He may be getting bored. He is probably feeling very confident as one of the older children (big fish in little pool syndrome). His behaviour may be completely different when he goes to school as it a very differnt environment. I also think youngest children are sometimes much , much more street-wise than their first born peers and are used to standing their ground. My youngest was very difficult... until she went to school. She discovered the boundaries at school and has been a model pupil at school apparently. I used to worry that she would be branded a bully... but she pushed the boundaries at school for about 2weeks.... discovered where they are and has been perfectly behaved in school.
Also... at 4, he probably won't remember the next day when he wants to push /hit someone the discussion you had the night before. I imagine there is only a short period of time at pre-school left. Ask the supervisors to deal with him ... firmly and immediately.... He will learn. I sounds like you are doing all the right things.