Gauge help

[Crochet by Evelyn]

So, I was crocheting the gauge for sweater that I wanted to make and the gauge was supposed to be 16 stitches wide and 21 rows tall (10 cm by 10 cm). The 16 stitches wide was indeed 10 cm, but for the height, at 10 cm it was only 17 rows tall. And now I'm not quite sure what to do because the gauge I made doesn't match to the one the creator made for the pattern. Also something to note: the yarn i used is size 4 whereas the one the pattern creato used is 3. I think that might be part of the problem i encountered but I could find the yarn they said to use in the pattern. Some help would be really nice.

[bgs]

If the yarn recommended is a smaller weight than what you used your swatch would have been smaller.  Things that affect guage are yarn weight, hook size, and your personal tension.  Here is an article about guage and tension.  If you cant meet guage by altering the above factors then you would have alter pattern by mathematically relating the number of stitches you need to make in order to get right size eg if its too short then increase number of rows. 

https://itsallinanutshell.com/2020/06/29/crochet-gauge-tension-and-the-golden-loop/

[Granny Square]

^ Agree with Bgs.  I have the same issue, my stitches tend to be shorter than most designers'.  In my case I'd been crocheting a long time before I learned of the 'golden loop' and kept reverting to my old ways, so I either add rows or sub a taller stitch to get something that needs to fit to work right.

Does the sweater have a diagram of the garment's dimensions -- armpit to hem, waist to shoulder for example of height dimensions?  Or, what I do for a sweater is measure it 'in process' against one I already own to see if dimensions look reasonable.