I don't think 4 skeins would be enough to make a scarf if you used the yarn double-stranded.* If it were me, I think I'd take back the yarn and exchange it for something in a bulky weight -- it'll knit up a lot faster. Also consider doing something besides garter. Garter stitch is very compact lengthwise, so it'll take you more yarn and time to get the same amount of scarf compared to most other stitch patterns.
*If you're wondering how I got this, I multiplied the given gauge by .7 to get the approximate new gauge when double stranded -- this comes out to about 4 stitches per inch. Worsted weight. According to this chart, you'll need about 380 yards for a 8x54 scarf.