The patches for Chapter IV are still available. In fact, patches are available all the way back to at least Open Beta.
However, as others have noted or discovered, using an older client installer, even one that's only one Chapter removed from the current one, can or will have problems updating. Not to mention taking hours (or days, or even weeks

) to install all of the patches needed to bring such a client up to date.
There are
41 patches in toto just for Chapter IV, though perhaps not all of them would be needed, depending on the specific Chapter IV installer. And there are currently
30 Chapter V patches.
What many people do is to get the latest set of installation files (once they're available) whenever a new Chapter is released, copy their data files to a safe location, then perform a complete reinstallation. I did manage to get away with updating my second installation (beginning @ Chapter II) and continued that updating through Chapter IV. But by that time, the old, patched code was getting a bit creaky. When Chapter V was released, I downloaded the then-new 5.0.0. installation files and reinstalled the game.
I'd recommend taking Kroagnon's advice and use the link in his sig for the GFLive-free 5.0.5. client download files.