Hi beikaboda, welcome to the forum! Great questions.
Yes, you will need to model in a margin for the slide , usually 0.3 - 0.5mm clearance is a good starting point. You may need to adjust the margin depending on the accuracy of the printer.
For the screw, do you want it to slide in easily, or do you want the threads to 'bite' into the hole? 4mm hole for a 4mm screw is going to be a tight fit, so make it a ~0.2mm larger or smaller depending on how you want it to work. For such a tiny hole, fewer segments are better - 6 or 8 segments should be plenty.
Hope that helps!