Confused on Ribbing Stitch Instructions

[CadMax]

I am starting on a sweater pattern - however, my knowledge of knitting is VERY LOW. The guage says to cast on 25 stitches and then to complete 28 rows with P 3, K 1 ribbing.

I am confused because the stitches ate a multiple of 4 plus 1 and
I also don't know if this means every row should be P 3, K 1 or if I should alternate between that and K 3, P 1. A site mentions a slip stitch, but does that mean slip as if to knit or as if to purl.

Any help most appreciated! Happy Holidays!!