I hate to be the bearer of bad news, but yes, you do want to frog what you've done and start over. Not only will it be too big by several inches, but you may run out of yarn. Do you do a gauge swatch, where you knit up a square and measure and see if you're getting the right gauge? They take a few minutes, but it saves so much work in the long run. So try a swatch in the 4.5s, and maybe even 4mms, and see what happens. By the way, if you get the 18 sts, but start getting the wrong number of rows, don't worry about it. You can make up for that by just knitting more rows, but you can't adjust for width. Oh, and I just thought of something-- you aren't knitting a scarf or a bag, are you? Because if you are, the gauge is pretty fairly irrelavant. But if it's anything else-- a jumper, hat, mitts, waistcoat, etc., it will be an issue. Sorry about this! In addition to the Thank You button here, they should have one that says, thanks but I hate the fact you're right!