Knitting back and forth in pattern in the round

Greetings! I am making a pattern called the Sleeveless Tunic/Dress from the Handknitting Association of Iceland. It is a sleeveless sweater knit in the round from the bottom up. I have reached the point where I am to place the live sleeve stitches on a holder, and soon I will cast on for the yoke (the pattern reminds me of an Ann Budd seamless yoke sweater).

Here’s where I am confused: the pattern says "sl last 4 st of rnd for first 4 sts of next rnd onto stitch holder (the sleeve opening). K 45, sl 8 st onto stitch holder (the other sleeve). K back 47. st.
Work 4 rows back and forth in st st (K right side, P wrong side) on back and front, sl first st of each row.

The next part is the yoke, where I cast on for the shoulders and continue in the round. But how do I go back and forth? Do I break the yarn and work from separate balls? Thank you for any help you can provide!!

–Marina

Welcome to the forum!
You could work the 4 rows on the back of the sweater by turning and purling back across the stitches you just knit, continuing for 4 rows. Break the yarn, attach it to the front section and work 4 rows. Or you could do as you suggest and work the back and front at the same time with 2 balls or 2 ends of yarn.

Maybe this pattern?

Thank you!