well, if you have a sprinkler going on just plain grass, it takes a bit of time before it produces new grass/flowers (I did a build like this), so perhaps increasing the amount of sprinklers may shorten the time when that bonemeal effect happens...
quickest way is to test it yourself. have one area with 1 sprinkler, and one area with 4 sprinklers. let both areas run for about 10 minutes, with both of them having a MFR harvester. afterwards check which area produced more drops. if the difference is very small, then using more then 1 sprinkler does not do much...