There are several reasons why running calves are thick:

1, after running, the calf muscles are congested and I feel that my calf is getting thicker. If you rest for two days and look again, your calf should recover and your muscles will have a recovery period.

You didn't stretch after running. Stretching for 10 minutes after running can effectively prevent the calf from getting thicker and make your calf line better and longer. You can search online for videos of specific stretching exercises. There are many, just pick one to do after running every day.

3. If it's 4000m, I don't think you can run too fast, and you shouldn't reach the level of anaerobic exercise, so the reasons for your calf thickening should be the above two reasons.

I hope it helps you. I also run every day.