When creating a backup plan for a VM in a Hyper-V host there is no option in Block Level backup. However since the first full backup all the other backups are differentials. If I wish every 3-4 difs to make a full backup how would that possible?
VM backups use block-level by default, so you don't need to worry about that.
Regarding differentials:
You can specify any schedule on the scheduling screen. The first scheduling step specifies schedule for diffs(e.g. 3 times a week) and then there's a separate schedule for full backup(e.g. once a week on Sunday).