We are a family of 4 - daughter turning 18 and wanted to make it memorable.
Due to travel restrictions in Carribean and weather permitting limited options , i am looking at Spain as one of the options ...
We only have 4 days for the vacation with other commitments, so need your guidance /feedback

We are not art loving and just want to see the city and have fun as a family... Adding to this we are also vegetarians so food options are also important.

What can we cover in 4 days in Spain?

It would help if you can get a direct flight to either Madrid or Barcelona..Both cities have more venues to enjoy than just museums.. parks, architecture, beach (Barcelona) , plazas , many possible day trips. Do some research.
Although Spaniards love their meat and fish, there is no shortage of vegetarian restaurants in either city.

Is 4 days your total travel time or will you have 4 whole days once there?
Where are you coming from?
Can you get direct flights to Barcelona, Madrid, Seville, etc?
When do you wish to go?
What does “fun” mean to you? Seeing sights like churches, walking in parks, shopping, concerts, tours, amusement parks, etc?

4 days in an unspecified period in a whole country with no information where you are coming from

Generally Madrid in the winter is cold and hot in the summer
Cities on the coast tend to be cool in winter and warm/hot in summer

Choose one city, stay there

Some more info would help

What to choose?
https://www.spain.info/en/