When crocheting, what does it mean to "3 double crochet in 4th chain from hook"?

do three double in the fourth chain

need the rest of instructions to tell what to do for the first three chains

You will skip the first three chains and do three double crochets (dc) into the fourth chain from your hook. It sounds like a shell pattern. If you can tell me the name of the stitch pattern, I can probably help you more. Feel free to PM me if you have any issues with it. Good luck!

ETA: The three chains you are skipping are the same as a ch3 (chain 3) which you would use to move your yarn up to work the next row. You won't do anything in those three chains. If your next row is done in dc, you will chain 3 at the end of this row, turn your work and follow the pattern.

Do I skip the first 3 stitches and do 3 double crochets into the fourth?

Yes. It will probably look funny for awhile but after you do the next few rows it will look better. Just watch your beginning chains, when you put more than one stitch into a chain, sometimes it pulls the next one really tight and you tend to skip counting it when you go to make the next set of dc's.Like if the next step is "skip 4 ch, 3dc in next ch" be really careful counting those 4 ch because one of them might be tiny now.

On row a million each time you labored chain 3 you created an area under the cgains. that's stated as ch-3 area. The development is telling you to insert your hook into that area under the chains and dealing(sc,ch3,3dc) in each area.