I think the "control" part is overrated at anything less than 4.5 level.
You can add more control with string types and tension. You will not be hitting the back fence. If you want to paint the lines at the 3.5 -4.0 level, you are going to play low percentage tennis which won't translate to winning at this level. Only valid reason I see for 18x20 at this level is if you prefer hitting a flatter ball or lower trajectory. Lower trajectory = more risk = more errors.
I think it depends on the person. I see a lot of 3.5 - 4.0 players hitting spinny, but loopy and ineffective groundstrokes. Some of these people would be better off hitting thru the court more. An 18x20 Blade will still give you plenty of spin providing you have the technique.
It is hard to make a recommendation without being familiar with someone's game.