Crocheting a blanket in a different size yarn

[Sunshine Crocheter]

I am going to make a lap blanket. It calls for 4 weight yarn. I accidentally ordered 3 weight yarn. Will it make a difference in the pattern if I use a  different weight of yarn? How would I addjust?

[bgs]

It will make a difference. 

https://www.allfreecrochet.com/Tips-for-Crochet/How-to-Adjust-Crochet-Pattern-for-Different-Yarn-Weight

[Granny Square]

Excellent article that Bgs linked above.  I just want to point out, the last line of the article is the first thing I would have pointed out--you may need more yarn if you want to end up with the measurement of the original blanket.  This is even more likely to be the case if you crochet at a tighter and shorter stitch height gauge than the designer of the blanket (I run into this all the time, not subbing yarns, but just trying to hit gauge).

Yarn sizes is a wraps per inch scale.  #3 is 11-15 wraps, #4 yarn is 9-12 wraps, so there is a little overlap (notice not all sizes overlap).  You should probably be OK yardage-wise if you stick to the same size hook the pattern calls for, which is going to make a looser fabric; you'll need to swatch to see what happens with your personal gauge.  If you want a tighter fabric more in line with your thinner yarn, you'll need to order more yarn, and make sure the lot codes match.