You will need to download 4.8, compile it separately and adjust the build process to use the 4.8 headers and libs.
I typically install a newer version and configure using --with-incompatible-bdb. Package "libdb5.3++-dev" or whatever's available.
It means my wallet.dat files won't work with clients that use 4.8, but I never use 4.8 and never share wallets, so I don't care.
It's a UX/UI issue. It's possible to lose access to a wallet by failing to back it up before switching to a later non-backwards-compatible client which, if for some reason proves undesirable to the user, causes them to switch back to the old client. That's when they discover that the new client automatically upgraded their wallet to the new db format which cannot now be read by the old client. That's why the advice (for distributed binaries, i.e. OS X/Windows) is to link against 4.8.
Great support, dooglus, Graham, et.al.
Is there a Clams TechSupport FAQ or Wiki currently, where these kind of tips and techniques are collected? I see similar types of questions recur. It would be great to pull this expertise into a ready reference for new users and investors. There is lots of expert info in the pages here, but not so easy to search.