It means to knit 4 sts together though back loops.
You insert a needle thought 4 loops starting with the 1st st on the left needle, then you knit these sts together.
I found a video that will give you some idea of how to do k4tog tbl. This video is only do k3tog tbl, but it is the same concept. You can just add a stitch to yours to make 4.
I hope it helps.