Does alt row mean to do the decrease every other row?
Yes, exactly. Calling your next row Row 1:
Row 1: Cast off 3, work across in pattern.
Row 2: Work in pattern.
Row 3: Cast off 3, work across in pattern.
Row 4: Work in pattern.
Row 5:Cast off 3, work across in pattern.
Row 6: Work in pattern.
Then you’ll do it again, but casting off 4 and working alternate rows 16 times (32 total rows) and ending with 4 stitches.
What is your pattern and designer, please?
The pattern is Nourish by Martin Storey. This is the instructions to shape the right shoulder, so do I do the decreases at the end of the row to be the reverse of the left?
Make the cast offs at the beginning of the row for the left shoulder. On one shoulder the cast offs will be at the beginning of a RS row and for the other shoulder the cast offs will be at the beginning of a WS row.
The difference of one row isn’t going to be noticeable.