Fire, Water, Thunder and Steel. That's a great combination of types.
Consider getting a Grass type. It may sound useless, but it'll work very well.
Also, you will need Fly. I'm not sure if any of those Pokemon will learn it, but either how you will need Fly. Try get a Grass-Flying type, like Tropius. Tropius is found at Pastoria Marsh without National Pokedex, and so it is possible to see it on your first visit. I think you may have passed Pastoria because you have already versed Wake. (I think you have anyway. Sorry, I haven't played Platinum in a while)
Of course, if you want a normal Flying type and to forget about Grass (Grass can sometimes become a little useless, however it's good when you need it, like there is no Water in the Elite Four or any other Sinnoh Gym, assuming I'm right and saying you've beaten Wake) you can settle for the classic Staravia (Staraptor is a very good Pokemon to have on your party, it can sometimes learn some strong Fighting moves like Close Combat, a great addition) but if you really wanna shine get a Drifloon. It may sound stupid, but trust me. The one time I played the game with Drifloon/Drifblim, I did not regret. It being Ghost as well as Flying is probably one of the best combos you could get, and it's available at a good level near Floarama Town at Lv. 21 (I think) so it's easy to train.
As for another slot, consider Psychic. (If you aren't gonna use Drifloon, Ghost and Psychic is just a waste of space) I haven't got any favourites there however if I had to choose one I'd choose Alakazam. Make sure you use an Exp. Share when training Abra, because for all I know, Teleport won't win a battle.
Consider Dragon as well. I mean, Dragon is actually quite good and can learn some strong moves like an old granny lives in the route before Celestial Town, and she'll give you Draco Meteor if she thinks you have a good bond with your Pokemon. For example, Gible is actually quite good. A Garchomp will smash anyone if you train it properly.
Also (I'm sorry about this wall of text. xD), Fighting can be a very good thing to consider. I can't suggest many, got Machamp can get your best bet there. Though it can be taken care of by some of your other Pokemon if you learn them the right Pokemon. If you can't trade though then I wouldn't worry about a Fighting type.
Remember, it's your party so you can choose.
These are just my recommendations. (Though, I would like to restate this, I think Drifblim would be an excellent addition to your party. A very useful, attractive and inquisitive addition to your party)
Good Luck on your journey! :)