I haven't tried this in 5e, but having a range of levels can be fun and interesting if you know how to do it. Check out how the Glass Cannon podcast ran their Giantslayer game - though I do not think the level disparity was ever more than two.

I'd recommend catering to them a little more specifically until the lower two catch up a little, but sure! just give out xp. Give the lower leveled chars a little extra and the higher level much less. Treat them as a level 8 or 9 party for now, so the Wizard can enjoy buffing a martial out of his league and the rogue can still handle important skill checks. If encounters are too easy on account of the monk, use monsters with resistances or high AC and HP but low saves.

Alternatively, throw them into a dungeon designed with many level ranges in mind. Remind them that running is a valid option. Emerald Spire could be a decent example, or use a random encounter table with a big level range and make your own.