There are 2 possible arrangements: 4,4,4,6 and 4,4,5,5
For, 4,4,4,6: divide 18 people in these groups: 18C4⋅14C4⋅10C4⋅6C6 seat them in: 3!⋅3!⋅3!⋅5!
(note that you dont have a choice in selecting a table, since all 4 are alike)
For, 4,4,5,5: divide 18 people in these groups: 18C4⋅14C4⋅10C5⋅5C5 seat them in: 3!⋅3!⋅4!⋅4!
What would be the final solution statement?