understanding a pattern

I am attempting my first baby sweater pattern. So far so good, however, I am to the point of shaping the armholes and I do not understand what the pattern is saying:

Shape armholes: Next row: Sl st in each of first 4 sts. Ch 1. Pat to last 4 sts. Ch 1. Turn. Leave rem sts unworked. 33 (39-42) sts.

Cont even in pat until armhole measures 4½ (5-5½) ins [11.5 (12.5-14) cm], ending with RS facing for next row. Fasten off.

I did the slip stitches in the first 4, that is as far as I could get I understand pat means pattern, what aI dont understand is what they mean by pat. Thanks for the help