이제 (Now) is used when something is started newly from before. For example, if you say 나 이제 배고파, I can notice that you were not hungry before and now you feel hungry. 지금 (Now) is just a pure Now with no implications to the previous situation. So if you say 나 지금 배고파, I don’t know whether you were being hungy before. I only know the fact that you are now hungry. The same slight nuance difference between them remains the same for 이제부터 and 지금부터. 이제부터 is From now (newly from previous situation) and 지금부터 is just From now (more time-focused), and it can be newly from the previous situation or not. We only focus the time.

Same