Janij - you never actually said how much rennet that you used the first time at 4 minutes or when you got an 8 minute floc time. In any case, I would cut the amount to 1/4 of your original (4 minute) dosage and see what happens. So if you used 1 tsp, then use 1/4 tsp this time. Do NOT add extra rennet during coagulation. The point is to see how your milk and rennet interact.
For Colby, I would use a multiplier of 3.5. So you add your rennet, wait for the floc time, and then multiply that times 3.5 to get your set time.
For example - If your milk takes 16 minutes to floc then 16 x 3.5 = 56 minutes. You will cut the curd at 56 minutes from the time you added the rennet.
FYI - just for clarity, most of us refer to "floc" time "set" time. Floc time is when your milk actually STARTS to set. The "set" time is when you actually cut the curd.