If I'm not mistake, PSIII actually takes place a few years after PSIV, meaning that Wren would have to do stuff in PSIV, then travel to a ship that's 1000 years away in 20 years, then do stuff there. Here's what I remember happening, from what background information I've found (please don't ask me to cite my sources. It's been far too long).
Wren series androids or automated production line created shortly before Mother Brain's destruction, allowing it to do their work through the great collapse. Shiren would have to be created before Mother Brain's death, so that he can be included in PSIII's ship (Alisa III, was it?), while Forren would have to start working in his satellite before the systems went haywire. 1000 years happen, twin Dark Forces attack, they both save their respective days.
So, short answer, they're different Wrens, they're similarly produced, they're roughly the same age.
Originally Posted by Omega-z
Well that's like saying Noah in PSI is different to Lutz through out the rest of the series but we know he's the same guy and he had different JP names too. It's the same with the Planet's names in each game too. What you need to do is follow the story not the names. and even then it could be explain away why the difference too.
There's a difference between original source. English translations are always different because they're translations of a Japanese source. I'm fairly sure the Japanese versions don't have this problem.
Originally Posted by Valinne
Is Forren and Fulyen Curtz a coincidence, or was that a deliberate reference?
Deliberate, just like the naming of the planets. There are a ton of easter eggs in PSU to the original series. Alot of the characters are based around classic characters, and Fulyen is one of them.