Hello Daniel and welcome to the community!
First of all we have two different Backup Jobs, one Daily incremental and one weekly active full backup job.
... I set the retention policy to 4, but the HDDs are running out of space because there are 6 Full Backups on the HDD now.
Have you set retention policy = 4 for both jobs? Do you have any full backups scheduled for the daily job?
6 can be a fair number of .vbks in your case. "Active Full job" is to have 4 fulls and "incremental one" may have 2 if it works an incremental method.
By the way, do these jobs backup same VM(s)?