They've already confirmed a 5th game, I think (it will be the last one starring Phoenix, though not the last one in the series -- I suspect they wanted to see the reaction to 4 before writing a definite ending to Phoenix's story.) This is Capcom, they never stop releasing games in a profitable franchise.
There are other series you might enjoy, although none with Phoenix Wright's courtroom drama/antics. (Well, except the Phoenix-Wright based Harvey Birdman game for the PS2, which uses almost the exact same system.)
Famicom Detective Club 2 is a mystery-adventure game for the SNES. That link is to a translation patch. It has a feel similar to the Phoenix Wright games, anyway.
The PORTOPIA Serial Murder Case is a murder investigation game for the NES. It's sort of the grandfather of all Japanese-style adventure games, including Phoenix Wright. Sort of dated, but still very fun. In addition to inspiring most other Japanese adventure games (you'll definitely notice the things Phoenix Wright's investigation sections borrowed from it), it also earned Enix the money they used to create Dragon Warrior. And you can play it on your DS on an emulator.
Snatcher (the SegaCD version was released in English and is very good) is another Japanese adventure game worth playing. It's sort of inspired by Blade Runner... you're a detective investigating murderous robots that impersonate humans.
I highly recommend all three.
Unfortunately, good GBA / DS mystery adventures are limited... there's the Touch Detective series, which is only ok; and there's Hotel Dusk, which is very different from the above games. Then there's that homebrew that lets you play graphic novels on the DS... there are probably some that would fit these requirements.