Cast on with 2 different size needles

I’ve downloaded this cardigan pattern from Ravelry. I’ve completed the back. I’m about to start the front left but the instructions aren’t making sense to me. So I’d appreciate it if anyone can help. The instructions says cast on 12 with size 5 mm needles and further cast on 50 with size 4.5 mm needles. So do I work with 4 needles for the entire front piece? and how?? Would it not cause a bit of a gap because of a needle dangling at all times while I work on the main patter? Here’s the pattern, if the forum allows me to link: http://s000.tinyupload.com/index.php?file_id=53322953227657679033

Otherwise it’s called An Eskevien on Ravelry.com.

Any help is appreciated.

Welcome to KH!
You’ll need to work the length of the first 12 rows of ribbing (the bottom ribbed edging or welt) with both sizes of needles. Then the pattern switches to the 5mm needles for the entire body of the fronts. So you’ll only be working with 4 needles or 2 circular needles for the length of the bottom ribbing.
There won’t be a gap because the same yarn strand will carry over between different needle sizes. You can always knit the stitches at the change over a bit more snugly if you’re worried about a ladder.