Another (lazy) way:
Find one of the Mondays you want highlighting, say, for example, it's in cell C10. Then in any other cell enter this formula:
=MOD(C10,28)
The 28 is 28 days=4 weeks = every 4th Monday.
Make a note of the result (Excel will probably auto-format the result to a date, you'll need to reset the format to General). You can now remove this formula. From the picture below you'll see I got the answer 23.
Now set up the conditional formatting as shown in the picture (the second CF, with the red font).

2024-11-22_120832.jpg

I also did the fortnightly Fridays where I got the answer 6 to the Friday I chose with
=MOD(C7,14)
being the first CF in the picture with green font. 14 is two weeks (every alternate Friday).