I decided to put a 4" travel fork on my bike as well. I used to run a first gen ('98) SID w/ almost 2.5" of travel. I liked how light it was but decided to change it to a SID 100. I found that my steering slowed down a bit, but overall, it wasn't that big of a change (probably cuz it sags 1" when i'm on it, and the older SID was made for XC racing w/ minimal sag). I like the way it feels now, but i doubt i'll go to 5".

And for the guy that was wondering why it would weaken your frame: i believe longer travel single crown (and double crowns) put far more stress at the head tube/down tube junction than a shorter travel fork.