I've been thinking of getting a 4TB external for backups (Pictures, Videos, Work..) since my a 1TB drive is no longer enough.

WD My Book
Does anyone have experience with this drive, or any other recommendations?
Very good drives, the best from my personal experience. I have one that's 500GB (basically was the equivalent of 4TB today) and still running, tried Toshiba and Seagate and this outlasted them.
I have used many WD passport drives and I find them reliable.
I don't have experience with this particular model (WD My book).
I used to get a drive and an enclosure and assemble them myself, but I believe the ones that are ready made (such as this one) are more reliable.
It's still a drive - it can fail - so if your data is critical you should keep a second backup.
I've been doing a lot of reading about the drive, turns out most of the time the usb connector is being soldered to the SATA (or something of the sort) making it impossible to connect the drive via SATA if the USB ever fails (same goes for the WD my passport Proof). Also, there's some sort of data encryption that renders the data useless if the board ever fails and you decide to connect the drive directly to the computer. Another thing is, it is impossible to know what drives are being used, there were reports of 5400 and 7200 rpm drives, green-red and several other drives inside.

In the end I decided I'll be going for a normal seagate 2TB 64mb cache 7200 internal drive since I have a dock built in to my case and considering the fact the drive will only be used for backups.