I need help with a pattern instruction

[allthingsfiber]

Please help me read this pattern.I am knitting a Double Breasted Vest from Lion Brand's free patterns. It is s st with a seed stitch edge. I have finished the back and started the left front but now I am totally confused by the following instruction for the Left Front--
V-neck shaping: Next row (RS) Work in St st to last 5 sts, ssk,
work 3 sts in Seed st.
Next row (WS) Slip 1 st, work 2 sts in Seed st, purl to end. Repeat
decs 16 (16, 17, 17, 18, 18) times at neck edge, alternating every
third and fourth rows. Note Dec will be on both RS and WS rows. AT
SAME TIME, continue as before until piece measures 13 (12½, 12, 12½,
12, 12)" from beg, end after a WS
I understand that the decrease that is being repeated (16 times for sz
small) is formed by this instruction:
Next row (RS) Work in St st to last 5 sts, ssk, work 3 sts in Seed st.

I have completed that row and the one that follows which does not
decrease:
Next row (WS) Slip 1 st, work 2 sts in Seed st, purl to end.

And then I am lost. I asked Lion Brand and their answer is "You are alternating between every 3rd and 4th row; that therefore means you will alternately be on the RS and the WS. Alternating every 3rd and 4th row means 3, 7, 10, 14, etc.These are odd and even rows so you are alternating RS and WS as well.Just be sure you work the decrease at the neck edge."

I'm still confused. My question is - alternating WHAT every 3rd and 4th row and how can you be on both AT SAME TIME and continue exactly WHAT as before until 13"? Can someone please reword this set of instructions, please, please. Thanks!

[suzeeq]

Repeat decs 16 (16, 17, 17, 18, 18) times at neck edge, alternating every third and fourth rows.

You're alternating the decrease row every third and 4th rows. The AT THE SAME TIME part means to continue in the pattern stitch I think, while you do the decs and shaping.

[allthingsfiber]

Does this mean that I will work in the stockinette stitch with seed stitch edge pattern each row (as in row 1,2) but at every 3rd and 4th rows (as LionBrand said, rows 3 7 10 14 etc.) I will decrease at the neck edge and therefore decrease at row 3 and 4 and again at 7 and 8 and 10 and 11 etc.???? Thanks so much for helping me figure this out.

[suzeeq]

That's almost right. You do the st st w/ seed st edges on all rows, and decrease at the neck edge on the alternate 3rd and 4th rows. This doesn't mean the 3rd and 4th rows, but to alternate them. So dec on the 3rd row (row 3), then the 4th row after that (row 7), the 3rd row after that (row 10), the 4th row after that one (row 14).

[allthingsfiber]

Oh, thank you, thank you, thank you! I really appreciate your help. Now I can get back to my sister's present. Wow, I was so bewildered by that set of instructions! You are the best.