Given the information that you provided and knowing nothing else about your physical condition...

I recently restarted cycling to work. I did four round trips. After riding back from the fourth trip, my stomach muscles started to tense and cramp.

You may want to consider some sort of torso (core) strengthening and flexibility routines. There are a number of ways to accomplish this. The methods include things like: traditional strength training, yoga, pilates, tai chi, and others.

Sounds to me that you're re-starting an activity that your body isn't quite ready for. So, maybe you just overdid it to begin with?

Given the information that you provided and knowing nothing else about your physical condition...

I recently restarted cycling to work. I did four round trips. After riding back from the fourth trip, my stomach muscles started to tense and cramp.

You may want to consider some sort of torso (core) strengthening and flexibility routines. There are a number of ways to accomplish this. The methods include things like: traditional strength training, yoga, pilates, tai chi, and others.

Sounds to me that you're re-starting an activity that your body isn't quite ready for. So, maybe you just overdid it to begin with?

After riding back from the fourth trip, my stomach muscles started to tense and cramp.

It's not certain that the cycling caused this. Sounds like your first 3.5 trips were ok; it's just the latest return trip that caused the tense and cramped stomach muscles? Unless this is a recurrent issue, I would not blame it on the cycling.