Nolo,
That is a very excellent sounding setup for the MWTs!
Would you be expanding that somehow to surround by adding amps and processors I'm assuming? If possible, I'd do that (keep with amp separates) rather than move to a surround receiver. That would most likely be a step backwards in terms of getting the most out of the OHMs.
I'm not a surround kind of guy, but if I were, I'm pretty certain I would prefer to build around OHMs and the wide range omni Walsh drivers in them. It seems a natural fit.
4.5 mkIIs are Walsh 5 mk II drivers (which I have never heard) on OHM 4 cabinets, right? I think this is the model that reviewer John Potis owned, reviewed, and praised.
Assuming this, from what I have read, yes, it should be a big step up for non-nearfield listening in a larger room in terms of impact and overall weight to the performance.
mkII drivers use a different tweeter than S3s I believe but the full range Walsh drivers are the same or mostly similar as I understand it. There may be a slight difference towards the top end as a result but mkIIs can probably be had for less. Its probably a very reasonable tradeoff, especially for home theater use.