R420 will not get the 5 pot. It will however (probably? maybe?) get the 420bhp turbo 4 that was previewed in the TT 420 Quattro Sport Concept. I also anticipate that the next TT-RS may get the 420 4 pot. They'll make it sound 'better' than the Golf R/S3. If you listen to you tube vids of the R400 concept, it does sound meatier, but then it could all be fancy sound play and the rubbish quality of iPhone vids!
We all know from the previous gens that Audi won't make a 3 door version of the RS3. They always said that if you wanted a 3 door version, buyers should look at the TT-RS. I can see the Saloon fit into the line up for the competitors like the Merc, so I really do think it will come.
If the TT-RS does take the 420 4 pot, then VW and Audi will have the 'same' flagship cars. There may be room for RS3 Plus as per previous version as 365bhp will be trounced very quickly.
Given the plaudits of the Golf R's chassis and handling (over the S3 I might add), I'd expect the R420 to be a very capable car indeed. Needs to look the part too though. OK, the added plastic F1 style aerofoilly things on the front bumper could look naff (like the Merc's), but VW have been very cute recently. The Golf R isn't shouty, but then again nor is the S3, the car it should be pitted against, not the RS3. If the ring spy pics are anything to go by, the rear roof spoiler is the only new enhancement and it's muted that it's only months away from a full reveal. It has the wider front track (like the RS3) and the wavy brakes (like the RS3), but I doubt very much it'll get the 5 pot. It needs to stand out from the Golf R and a simple rear roof spoiler won't do that. It has to take cues from the R400 concept with the flared arches, lower ride height and some appendages.
What is true is that things are getting very hot in the hot hatch arena. Waiting until the end of the year and you should see a raft of new motors which will be very hard to choose from. R420, TT-RS, RS3 Plus maybe? RS3 saloon (and Plus for that matter). And then there's that darned M2 to think about. There's even rumours of the M2 CSL prototypes running around the ring with the M2 mules. I need to wait to feed my excitement and to arrange for all the upcoming test drives!