I use FF4 on Slackware - it seems stable, and hasn't crashed yet. The rationale behind the BLFS book is to provide a set of instructions that will give you a usable system, they're not trying to give the the latest and greatest. So I would actually have been surprised had there been information on compiling FF4. I would go for FF4 if you can/are willing to, because personally I find it an improvement to FF3, but I don't think it will make much of a difference to your system either way
