Question about the exemptions on a paystub?

When you receive your pay stubs and there is a number in the exemptions line or number of allowances. That means your dependents and you. Correct? Ok so all my stubs always had 3 ( for me and my 2 kids) I just got a new employer and got my first check and it has 4. When I was filling out those W2 papers they give you. I checked 1 for single, 1 for head of household, and 2 for dependents. So is that why there is 4, and can I get in trouble. Its just confusing cause I always just saw 3 and I filled head of household last year and there was just 3. So what do I do? can that be fixed?