It is not at all a dumb question !
I can tell you how I would go about this with a two blade easel. I don't know your 4-in-1 . . .
First you set it up simply using your eyes and the lens wide open. You focus and you adjust the height of your enlarger. And you set your easel's blades for this image. It can help to turn off all darkroom lights. Then I bring down the horizontal blade, which should create the even surface you need to put your grain focuser.
It can depend on which grain focuser you use and on which enlarger. Sometimes you need to push aside a redfilter or filterholder because they're in the way. If all fails you could consider a different lens, like a 60mm instead of a 50mm. If you use an enlarger with automatic focus that can prove to be difficult, non automatic enlargers are easier in that respect
3,5 X 5, is that inches or cm?