Here are some questions that will help narrow down your answer. How many headstones? Are you responsible for removing flowers and other memorial items people leave? How neat is the placement of the stones? Are they just place wherever the family decided to place them or are they neat and in straight rows?
4 acres with the 52 you should be able to bust that out in well under 2 hours, depending on obstacles and the lay of the land maybe down around 1hr15min but that is pretty much topping out productivity with the mower and averaging around 9mph.
Trimming is where you are going to run into your biggest time killer. Like I said previously, w/o seeing the property and knowing how many stones, how they are laid out, trees, other obstacles, fences, sidewalks, beds anything else that really kills the ability to knock it out fast it is hard to estimate. I would venture to say you are looking at minimum an hour of trimming and maybe up to 3 so that is somewhere in the neighborhood of 2.5 to 5 hrs for this job.
If this is a 4 acre cemetery, packed to the hilt with stones and the church and the parsonage have no real yard to speak of then I think your time is going to be way different than what I am imagining this setup to look like.