I own a Road King 1 and a Mark IV.
The Road King has the same preamp sections as the Roadster for Channels 3 and 4.
The Roadster has Better preamp sections for channels 1 and 2. The Roadster reverb is better as well.
So from a clean channel persective, the Roadster is much better then the Road King 1, which makes the clean channel comparison between the Mark IV and roadster even easier since the RoadKing 1 has better cleans then the Mark IV
Mark IV has a very good and usable clean channel, but the Road King clean channel is noticably better. Given that the Roadster has an even better clean channel and better reverb then my version of Road King, I can only say there would be a big drop off in the clean channel! Very big!
Leads on the dirt channels. Here again the two amps are not comparable. Mark IV is better. Not really close. Mark IV is much smoother, better sustain, more natural compression.
The Mark IV is to midrange as that the Rectos are to low end. I think this really assists the Mark IV in sounding great for leads as the notes sounds much fuller to my ear.
Mark IV is a very tight amp. Really does well with the muted palm stuff. You just need to keep the bass on the lead channel at 4 or below. If you think there is not enough low end, just add it back with the built in EQ.
But it won't ever get as much low end as the Roadster. You will miss that Recto Growl and wall of distortion.
If you are looking for the drop C chugga chugga type stuff and want amazing cleans, it's the Roadster by a mile.
If you are looking more for shredder type leads and reall tight response and can live with a good sounding clean channel, it is the Mark IV.
The Mark IV can really get a good range of tones, but as another poster points out, unlike the Roadster, they may not all be available at the click of a footswitch.
While the Roadster is versatile, it is a Recto, and will always sound like a Recto. To most of us here, that is a good thing. But you already own it and already know this.
And going from 4 independant channels to really 2 and a half independant channels because of the shared partial controls between R1 and R1.
Check out a couple of my favorite Mark IV youtubes
On the lighter side, with a Strat
http://www.youtube.com/watch?v=N9Cgem1m94k
Some guy doing his own version of Master of Puppets
http://www.youtube.com/watch?v=FpWULOhw5lo
Regarding Price, here is what I got from Ebays completed listings
Mark IV heads that sold, average price was 1490. If I throw out the highest (1700$ for one with blond tolex) and the lowest (1000$ for a rackmount Mark IV), it is about 1580$
The Mark IV combos went for much less, the two that sold averaged about 1100.
The Roadster, only saw one sold and it went for 1700.
I think you could pull some extra cash in.
The thing is these are two very different amps.
I love my Mark IV. It is amazing.
But if I already owned a Roadster, I don't think I would give it up for a Mark IV. I would just save my *** off!