Help! Fingerless Glove Pattern - Thumb Gusset

This is my first glove/fingerless glove and I’m stuck on the thumb gusset. First, how many needles should I have - I have 3 doing the circular knitting (total of 4 needles), but should I have 4 needles for the circular part (so 5 needles?).

I’ve completed the neck warmer and am working on the wrist warmers. I’m stuck on the thumb gusset. Can someone explain what I need to do for the increase? I’ve changed back to the larger needles and have knit the lace pattern for 20 sts. - it says to “Place marker, m1 between needles 2 & 3, place marker, pattern to end of round.” - does this mean put two markers together next to each other, and then I’ll be doing the increase in between the markers?
I need to finish the wrist warmers for my sister’s 40th!

Wrist Warmers
Using larger DPNs, cast on 40 sts, place marker and join, taking care not to twist.
Purl 1 round
Knit 1 round
Purl 1 round
Work rows 1 - 8 of lace pattern twice.
Change to smaller needles and work rows 1 - 8 one time.
Change back to larger needles and prepare for thumb gusset.
Continuing in Lace Pattern work 20 sts. Place marker, m1 between needles 2 & 3, place marker, pattern to end of round.
Rnd 1: Pattern 20 sts. slip marker, knit to next marker, slip marker, pattern to end.
Rnd 2: Pattern 20 sts, slip marker, m1, knit to next marker, m1, slip marker, pattern to end.
Repeat rounds 1 & 2 until there are 15 sts between markers. (Adjust new sts evenly between needles 2 & 3)…

Whether you work with 4 or 5 needles total is up to you. I use 5 but lots of people prefer 4 needles and the patterns work just fine.
When you get to the end of needle 2, place a marker. Then knit into the thread between the sts on the row below and place a marker (now you have 1stitch between the markers). Continue with the pattern, increasing on the rounds it says to, between the markers until there are 15sts between markers. If you’d like to see the increase, you can look at the Free Videos tab at the top of this page, Increases.
It’s a great pattern for what will be a lovely present