I just replaced the plugs in my 97 4.0 and the old plugs were gaped at .085 and the spec sheet said they were supposed to be .035.

Also they were platinum and i replaced with copper since i have read that is what is best for these engines.

What is the down fall of the plugs being gaped so much?

Will my jeep run better now i have the correct gap? It wasnt running bad but it eating gas.

Either someone never gapped the previous plugs or they were in there a real long time. Downfall would be hard starts, misfire. You should get easier starts smoother running with correct gap.