AMELIA is the product of a partnership between NASA and the California Polytechnic State University. One of its innovations is circulation control. An engine is mounted over each wing, and high pressure air is redirected from the exhaust over the wing and through tiny slots, adding lift.
AMELIA is still in wind tunnel testing, but if it makes it to the prototype stage, it would have a carrying capacity of about 100 passengers. A good commercial comparison might be a low-seating version of the Boeing 737, although it's old. A newer (and perhaps better) comparison is with the Airbus A318. Its cousin, the A319, is a similar aircraft that has been more popular.
Anyway, assuming that a prototype/production version of AMELIA is manufactured, it might be pretty similar in size to these aircraft. They're all twinjets; AMELIA, though, has its engines on top of its wings. In theory, one could modify AMELIA to have more engines. This version would be different, just as the 747 is a completely different craft that the 737, or how the A318 can't compare to the A380. Scaling it up, though, could be possible.
Would the addition of two more engines1 interfere in any way with the airflow from the original engines and the subsequent lift produced? I doubt it, because engines on four-engine aircraft (e.g. the 747 and A380) don't seem to present any interference issues that impact performance. But the setup for AMELIA is pretty unique. Could adding an additional engine on either side interfere with airflow?
Image credit: NASA.