Yes, you should have 4" over 18 sts, but cast on about 24 and don't use the edge sts to measure, knit about 3-4" worth. Usually row gauge is not as important because most patterns will tell you to knit to a certain length, not how many rows. These are top down patterns and many knitters will tell you that row gauge is important there, but the designer must feel that if you get stitch gauge, you'll be fine on the rows.
The other thing is -- almost no one gets both stitch and row gauge on any pattern, so don't worry about it.
sue- knitting heretic