I think flat is just fine. As for the height, or action, that's usually left up to player's preference. .
Most three and four string cigar box guitars have necks that are flat and straight. String height can vary quite a bit depending player preferences , but most cbg's have strings that are high for use with the slide. Keep checking out you tube videos and the forums here at cigar box nation.
I would agree with the others. I have built and played plenty of 4 strings and have found a flat fretboard to be just fine, as long as the neck is not too wide to comfortably bring your fingers around.
That being said, I have recently bought myself a 16" radius sanding block, which I will be trying on my next 4 and 6 string builds.
As for string height, I tend to go for low enough at the nut to intonate perfectly at the first fret, and about 2.5mm at the 12th.
If you can be bothered, I would recommend tapering the neck though!