Nthing the recommendation of a saddlebag. A backpack will be a real drag after a few hours; I can't imagine wearing one for four days on the bike. When I tour I use panniers. But for long day rides I often use a Carradice-style saddlebag. You can pack an amazing amount into one of those. If it's big, you might need a support to keep it from rubbing against the top of your rear wheel.

You should take a good look at your bike before leaving, and repair or replace anything that's worn. In addition to your normal flat repair kit, I'd suggest bringing another spare tube, a patch kit (if you don't normally bring one), and a pump (if you normally use CO2 inflators). Your group should have at least one minitool with all the tools you'll need, a pair of needle-nose pliers for extracting wires from tires, a few tire boots (I like the Park Kevlar ones, because the adhesive makes them easier to get into position), an extra master link for each kind of chain, a spare brake cable, a spare derailleur cable, a FiberFix replacement spoke, some zip ties in assorted sizes, a small roll of duct tape, a small bottle of chain lube, and maybe a small tube of all-purpose synthetic grease. I also like bringing a few pairs of disposable nitrile gloves and a few individually-packed Wet One antiseptic cloths. A small hiker first-aid kit is also useful. Finally, consider a replacement rear derailleur hanger (Problem Solvers makes one; there are others on the market too); it's light and could come in handy. You're not likely to need this stuff, but it's handy to have if you need any of it, and it doesn't take up much weight or space.

Be minimalist with off-bike clothes. I like lightweight nylon convertible pants--the kind where the lower legs zip off to convert to shorts--and a lightweight synthetic collared hiking shirt, which gives a look that's casual but more elegant in a restaurant than a t-shirt. Ex Officio makes nice lightweight, quick-drying underwear. The idea of shipping stuff ahead is also intriguing, especially if you could also ship a prepaid priority mail box with it, so the hotel could send it back. But if you're not sweating in your off-bike stuff, and you shower after your ride, you shouldn't need to wash it daily, if at all.