Its not your mess or your expense, Change it or forever be cutting down foam cushions when you want to change up the look.

Way too high for comfort. This is fundamental design consideration. Maybe there was a rush or other circumstances that caused this mistake. Common problem. I would discuss with designer and craft for fix. Maybe they can share expense?

The cushion doesn’t stay 4” when you sit on it, and a 2” cushion is going to feel like nothing after sitting for a few minutes.

I think this is a question for the designer to answer . I agree a 2" cushion when sat on will feel like nothing. I need to see the issue but just shy of 18" mwans what ? A 4" cushion will sink down when sat on so how much leg room do you have at the table when you are seated . Maybe the table is too low or has an apron thta is making the seat feel too high. I have had dining chairs that were a bit higher than 18" but were not an issue with my table. I need pics of thetable and the seating.

I usually do 3" cushions for banquettes, but even then that will be too high. The seat should top out at 20" including the cushion. This is your designer's mistake. S/he has to come up with the solution. On her dime, IMO.