Rather than thinking of debian in terms of version numbers, you need to start thinking in terms of release stages. Don't think "Debian 3", think "Debian Stable". The "Stable" (aka Woody) means that every package you have access to has undergone significant testing, and installing any given package will almost never break your system.
Pro: You can automate upgrades and get the latest security fixes without worrying that something will break.
Con: It will take months from now before php 4.3.x will appear in Stable, if it does at all (enough changes have been made that might break existing .php code... you see where this is going, right?)
So, what is there to do? You'll have to get php from a higher stage. "Testing" (aka Sarge) is the next step up. According to http://packages.debian.org/testing/web/
php4 is at 4.3.3
Pro: Newer packages than Stable
Con: *no security fixes*
If not having up-to-date security fixes scares you (and it should), you can upgrade to SID instead. SID stands for "Still In Development" and consists of the newest packages in Debian. This is the "Unstable" branch. Here, we have php4 4.3.4
Pro: Newest packages possible. This includes security fixes
Con: Newest packages are often untested and can break, sometimes seriously.
Still want to upgrade? Edit /etc/apt/sources list and replace your deb line with
xxxxx main contrib non-free
where xxxxx is stable testing or unstable. (and .us. is your country code, in case you don't live here.) apt-get update; apt-get dist-upgrade
Don't want to upgrade? You have two choices. 1) Search http://www.apt-get.org
for a "backport" of the php4 package you want and follow their instructions for installing it. 2) Backport the package you want yourself. The easy way is to follow these steps:
1)Choose which stage you want to backport from (probably unstable) and create the line in /etc/apt/sources.list (comment out any existing deb-src lines).
xxxxxxx main contrib non-free
2) Install all compilation tools you'll need (the meta-package "build-essential" will automatically install these)
3) run apt-get source --compile php4 This will download and attempt to compile php4 into packages. If it says you need yyyyy:
a) try apt-get install yyyyy. If it works and you can get the right version for the php package, try compiling php4 again.
b) if it doesn't work, apt-get source --compile yyyyy.
---- Repeat the above steps until you can get everything you need compiled and installed (the --compile step will create .deb files you install using dpkg -i filename.deb)
The hard way (which is what is used to get on apt-get.org) is to pick apart the configuration for the newer package to see what will and wont work on the stable system. Disable features where the libraries are not available, change the configuration to require lower version numbers, etc.
In your case, apt-get.org lists several php4 backported packages. This is the way I suggest you go.