For a four day credit card tour, you won't need that much. Definitely not 25 litres, unless you're doing it in the middle of winter at the top of Norway.
I've credit card / youth hostel toured for up to two weeks with a Carradice Nelson (14 litres), it worked fine.
here's my kit list from one such tour. One of those, a rack pack or an Alpkit Koala type of big saddlepack would all be perfect for your needs. Travel light, you won't regret it: bikes are such less fun to ride when they're laden down!
FYI, I'd probably buy a Koala or similar if I didn't already had the Carradice, but it depends what you want from it. Here are the pros and cons:
Good points about the Carradice:
It's very well built and is as tough as old boots.
The Bagman QR makes it very quick to attach and release from the bike
The side pockets are easily accessible while on the move.
You can cram stuff in and cinch it down with the lid.
You can easily bungee stuff to the top.
You can easily attach a shoulder strap.
Bad points about the Carradice:
It's not the lightest of contraptions.
It's probably not the most aerodynamic of contraptions either.
Together with the Bagman, it's quite expensive.