If you take maybe 5.4 seconds to look on palmgear.com you'll see a sh*tload of editor that can handle files as large as you like, the 4k limit has been breached a looong time ago. Why aren't there PHP editors for palm? well why would you want a PHP editor if you don't have an apache to run PHP on? can you get mysql/postgreSQL on a palm? nope. so what's the point of having a PHP editor? 🙂
I do my 'on-the-move' editing on my laptop, much more easier than trying to tap the right character on a palm. And it has a nifty IRDB interface to my phone.
Get yourself a Toshiba Libretto, a little larger than your PDA, but a sh*tload more functionality (you can even run linux on it)