The pattern says purl 3 cable, 4 back cable, 4 front. Please explain.

It seems to be saying that you purl 3, followed by a cable 4 stitches, by slipping stitches #1 and #2 to a cable needle, hold 2 stitches on the cable needle to the BACK, knit the 3rd/4th stitches and then 1st/2nd stitches off the cable needle and then proceed to cable the next 4 stitches, by slipping stitches #1 and #2 to a cable needle, hold 2 stitches on the cable needle to the FRONT, knit the 3rd/4th stitches and then 1st/2nd stitches off the cable needle. Cable knitting instructions are often confusing, especially if the writer doesn't use 'standard' knitting terminology.