Installing 6's vs 4's won't alleviate the problems we see with heads being pushed down into the ground. That has happened either because of improper compaction below the initial install or due to something heavy smashing the thing down into the ground. I see this from time to time, but it's not common. Usually somewhere they had a heavy truck drive over the curb, etc. For all turf, bermuda and fescue (what we have here) we always use 4" and install them at ground level and properly compact the soil underneath them before tightly packing the soil around them. We have never had any problem with the spray being obstructed by the turf if the turf is kept at a healthy height. We primarily use 12's for beds. very rarely use 6's.