Well honestly your team looks pretty solid Pokemon wise going up against the E4 especially once you evolve Haunter and Kadabra so you might just need to power level them up a bit or change their movesets if that is the issue?
As for your 6th member I have always been a fan of Primape and Hitmonlee since they both pack a rather nasty punch as well as being relatively fast.
Other options you could go with would be Poliwrath (decent Pokemon and fun to use), Snorlax (common and can take a lot of punishment before going down), Nidoking/queen (decent stats and good coverage moves), Rhydon (hard to capture and takes a while to lvl up but strong and bulky), and Vileplume (overall solid Pokemon that can status the E4 Pokemon to make them easier to handle).
Really though just having something in your 6th slot with help out a lot against the E4 and there are plenty of decent options for it. Just find what you like using best and go for it ^^