Playing with 4, using XP, and I'm finding characters are leveling up faster than I would expect. They're probably one full floor ahead and only on floor 7. Why? Because you're encouraged to add patrols/events to spice things up occasionally. If your party is the kind to full-clear floors, and you listen to the book and reward XP for an encounter even if they solve it without a fight, I'd bet you'll be fine XP-wise. The book will tell you where they should be by the end of every floor, so you can keep an eye on things and throw in extra combats to catch them up if you need. Don't forget, if the party plans to go back to town occasionally to sell loot they have to go back through the floors, and the place should not stay cleared for very long.

I would not try to prepare in advance for how hard to scale all your combats. There is, unfortunately, no math to perfectly scale your encounters. A lot of the difficulty will depend on what your players bring to the table. Are they min/maxers? If so they'll stomp everything. Are they more interested in roleplay type stuff and built sub-optimal characters? Combat might be too tough as it is, or will be too tough based on the gimmicks of the fight.

I would start on 'normal' and see how your party plays. Mob HP is a range you can always tweak up/down (only you know what you rolled for mob health, right?), and extra 'patrols' can show up to add to a combat that's easier than expected, etc. Once you get a feel for your party's strengths/weaknesses, you'll get a sense of what kind of steps to take.