A couple of

[jack]

Are these correct? If not, why? What do they mean?
1. He works a couple hours a week.
2. He works couple hours a week.

3. He works a coulple of hours a week.
4. He works coulple of hours a week.

[Casiopea]

Are these correct? If not, why? What do they mean?
1. He works a couple hours a week.
2. He works couple hours a week.

3. He works a couple of hours a week.
4. He works couple of hours a week.

1. and 3. are correct. 'a couple of' is a set phrase; 'of' is often omitted.