Before changing to a spray head watch this
https://www.youtube.com/watch?v=iCJtmcH0j-w If an area is not getting enough water lengthen the run time. Zoysia grass typically needs about 1 inch of water per week. St. Augustine generally requires more water than zoysia—about 1 to 1.5 inches per week. Use a rain gauge or tuna cans (cat food cans) to measure how much water your sprinklers are delivering to an area.
The Hunter MP heads and the Hunter spray heads both have a filter in each head, They should be cleaned at least once a year. Just pull the pop-up part up, clamp it, twist the head off and find the filter rinse it and replace. You tube has videos on the process. I installed a system filter as someone else suggested. You tube also has videos on this.
If you are thinking of going from the 4" heads to the 6" there is a potential problem. The body of the 6" pop-up head is about 8 1/2' long that is 3' longer than the body of the 4" pop-up head. So if you want to raise the body an inch look for a different way.