AV receivers can be configured to run with 4 speakers and still do surround. The most logical layout is Fronts L&R, Surrounds L&R. The centre speaker which isn't there physically gets replicated in "phantom mode". That is to say that the amp steers half the centre channel to each of the two front speakers. With the right sort of front channel speaker system and a room that's nicely damped (lots of soft furnishings and plenty of bookshelves and other stuff to break up the large flat expanses of wall) then this can work very well indeed.
What I would say is that ceiling speakers probably aren't the right sort of front channel speakers. They're firing down rather than straight forward for a start, so the phantom centre image is pointing at the floor rather than the listener. The other thing is "in-ceiling speakers" covers a lot of product. At the top end there are some sublime products, but OMG, they're not cheap. I'm talking £1500+ per pair. Even within a decent manufacturers range there quite a steep performance change from the best down to the entry level.
The final thing is bass. It's hard to generate the sort of convincing sub-bass from in-ceilings or even from conventional bookcase/standmount speakers. For that reason most home cinema installs also include a subwoofer. There are conventional stand-alone subs such as the MA Bronze W10 or BK Electric Gemini/XLS; then there are the in-wall subs designed to accompany in-ceiling speakers. The price/performance ratio isn't as good as the standalone subs but at least they're out of sight.
I completely understand the desire to keep things looking minimal. That's fine but it does involve some trade-offs that have to be understood if the final result is to meet expectations.
Perhaps there are some other products that offer a cross-over solution between minimal visual impact and sonic performance. Monitor Audio makes in-wall and shallow-depth on-wall speakers. Have a look at the MA
Soundframe range of speakers. KEF goes even further. Their
T series on-wall speakers have a clever driver design that means they can make a finished speaker in its cabinet at just 35mm deep. I've used the KEFs (T201 and T301 series) and they do sound remarkably good for a speaker thinner than an M&S prawn sandwich.
Both manufacturer's ranges include centre and surround speakers as well as the conventional but thin stereo front pair. It would be possible then to flank the TV with L,C,R or just L & R with none of the bulk of a conventional box speaker, but more effective cinema presentation than a typical background music general-purpose in-ceiling.
The other possibility is to mix shallow depth front speakers with conventional in-ceilings for the surrounds. If we are talking about mix 'n' matching then I'd be very tempted to go for a conventional sub rather than the matching slimline units for these ranges. Subs can be hidden under tables, behind sofas etc. Spending double or more than what you need to for a sub that's still only half the depth of something conventional doesn't seem like sensible planning unless space is at an absolute premium.
Oh, and bear in mind with in-ceilings that as much noise travels up in to the ceiling void as it does in to the living space below. If your friend is in the penthouse then that's not an issue. But if there are neighbours above....
That's covered most of the basics about discrete speakers and probably in a bit more depth than you were expecting. The next thing to talk about is the room itself.
