(Linwood New Jersey USA)
Just starting knitting again after 6 years. Picked up a sweater project that I started and am finishing the back.
The pattern tells me to cast off 4 stitches then leave the rest of the stitches on a holder (to shape the underarm). What do I do with the existing ball of yarn that will still be attached?
You can either break the yarn (leaving a reasonable length as a tail for re-join or sewing in later) or you if you have enough yarn spare so that you do not need it elsewhere for the moment, you can leave it attached just in case you need it there later.
I really can't advise which would be the better option without reading your pattern and to gain an understanding of the shaping up to and after this point in the pattern.
Hope this helps...