Ok, I'll see if I can explain it to you.
I assume that the 17 inch shoulder span is the example given in the pattern on how to work out the number of cast on stitches and the 22 inch shoulder span is your measurment.
If so, then you are correct in your working out that you need 30 inches in total for the 2 sleeves and the back of the bolero and that 30 inches at 4 stitches per inch is 120 stitches. As you need to round down to a multiple of 4 + 2 and 120 is exactly divisible by 4 then you can either subtract 4 from 120 to keep a multiple of 4 and add 2 to get the +2 or just subtract 2 to get your cast on number
120 - 4 = 116 + 2 = 118
120 - 2 = 118
An English Knitter (nationality and method).
Feel free to call me either Clutter or Julie (my real name).
FO: Downton Hat
A sweater of a thousand yards begins with a single stitch. Lao-tzu's Granny