Will I have to redo flight time for switching to a new CFI?
You will not need to repeat the hours. I flew with two different instructors during my training due to my first one heading off to the airlines and it was not an issue, as a matter of fact its pretty common. The FAA considers flight time, flight time. Once you logged the hours they are yours for the rest of your life. 

However the new instructor will most likely have you repeat some of the stuff you went over with the first CFI just to see where your skills are at. If you only have 4 hours under your belt is likely you have just done turns, climbs, decents, and possibly stalls so switching instructors will be no issue. It takes a fair bit of time to build a rhythm with an instructor anyway.  

Just some advice, if you are at a flight school that has a few others to pick from its worth it to ask them what their intentions are. Most instructors are there to build hours towards ATP then its off to the airlines. You should ask instructors how many hours they have left if they are going to ATP out, the more hours they have the longer they will be around. This will help you prevent the situation where you have to change instructors again.