Watch out for these, there are some things in them that seem to cause scsi troubles and some odd problems, i never could pinpoint it,but going back to os3.9 install with just BB1&2 seemed to fix it. I would suggest making a os3.9 backup to some drawer on the HD with just BB1/2 to fall back on in case then installing 3&4 on the main partition.
When I did mine I did a completely fresh install. Made a 3.9 recovery floppy and booted off that, formatted my hard drive then threw in the OS 3.9 CD and installed it, then BB 1-4, one right after the other, all in a row. Most of the times I've heard people having problems with 3-4 is when they install them on top of an already running system, with who knows what stuff already installed and configured. But yeah, I should have included a disclaimer: "your mileage may vary".

My .02 cents, I think it's worth the risk if you want "the latest and greatest" 3.9 system you can have. But it never hurts to make backups along the way.
