First sweater and confused - joining

This is my first sweater. Before this, I’ve done a couple of scarves and a hat using a circular needle (easy easy stuff). So anyway, this website has been able to show me a bunch, but I can’t find anywhere how to do this:
“Next row, knit all sts to neck opening, cast on 4 sts, join as one piece again and continue knitting on other side.”

In the pattern I’m working from, I am working from sleeve to sleeve and only need to sew the underarm seams. I’m in the middle of working both sides at once (each with its own ball of yarn) but I can’t possibly think of how you’d join them and continue to knit without cutting one yarn and just weaving it in or something. But that doesn’t seem very sturdy. Is that what needs to get done?

Any help would be greatly appeciated!

If you’re using two strands now, then yes, one of them will be cut and woven in. Sturdiness won’t be an issue–all the knitting will hold the sweater together just fine.

Thanks Ingrid! I keep doubting myself and thinking of insane ways to solve the problem at hand. LOL!