Casting on over gusset gap

I’m knitting mittens and after placing my thumb stitches on scrap yarn, the pattern says to use backwards loop method to CO 4 sts over gap. Do I just place these on the right hand needle after placing the thumb stitches on scrap yarn and continue incorporating the 4 stitches into the established pattern until I finish with the thumb? At the point where I start to knit the thumb, the pattern says to transfer the gusset stitches from the scrap yarn to two dpns; join yarn and knit to gap and at this point knit the 4 sts into sts cast on at top of gusset. I guess the part that confuses me is where it says to place the 4 stitches ‘over gap’.