"Does this mean I knit 6 rows then dec"
Not quite, work 5 rows and decrease on the next one, the 6th row.
"then knit 4 rows then dec"
Again, only knit 3 rows and decrease on the next one which will be the 4th.
"then knit and dec every 2nd row until it comes to an end?"
Yes, decrease every other (same as every 2nd) row, until you're down to 4 stitches.