There are 20 people and 14 tables. So there are two parts of this problem. a) On how many ways can these people sit at the tables, if none of them is alone. b) How many ways are there to make 4 groups (order is important) if there must be at least 2 people in a group.
I'm thinking of using inclusion-exclusion principle, but am stuck at formulating it, any help would be appreciated.