The new one is gen6..
Will I experience significant improvement in the performance of a new AirPort vs. my old 4th generation model?
Wish we could answer this but in reality I cannot say.
Wireless is a whole bunch of voodoo.. and in voodoo there is nothing scientific.. just beads and rattles.. and offering chicken livers to the gods of ether.
If you have AC clients ie very new stuff.. it will be faster in the same room.. if you have the same old N wireless devices there is a good chance it won't work any better.
Or anywhere between the two.. buy one and if it doesn't improve your network performance return it.. apple stores give you two weeks return.