Mp2617 sys voltage

Could sys voltage exceed programmed value? During charging, after, or when battery is removed or under any other conditions?

Is this just a fear you have? Have you seen something funny? If I were trying to break it, I would pop the battery out of the circuit just before the CC/CV taper begins with the Sys relatively unloaded. Datasheets rarely make explicit the problems of a chip. The Sys voltage can be as high as 4.6V so anything you hang on there has got to be 5V rated ( I would suppose) which usually means 6V abs max. That is quite an overshoot.

I want to use it to power gsm modem, whose working voltage is 3.4-4.2V. I hope that no additional regulators are needed.

During charge the Vsys is nominally held 200mV about the battery voltage 4.2V + .2 is 4.4V

So outlook is doubtful. You can override that, by programming Vsys to be 4.2V the problem then is that the battery will always be slightly undercharged and charge slower than is possible, that might not be a huge deal in your application. As I understand it a Li-ion battery charged to 3.8V is already at 83% of full charge. So you are leaving less capacity on the table than one might suppose.