For a bar I would suggest using commercial speakers rather than home models. http/www.jblpro.com/www/products/installed-sound/control-contractor-series#in-wall
They will take abuse much better.
If you are trying to get an even level of sound more smaller speakers is better than fewer large ones. The advantage of the large ones will be more bass and higher maximum volume so it depends on the type of bar you run. If your patrons make a lot of noise then you need to get the music louder. You might want more volume at the bar proper and less at the tables? You can connect passive volume controls to get more control.
Most subs are self powered and the amp does have a line out port for connecting a sub. The 8ohm speakers would be compatible with the amp even when running all 4 speakers simpultaneously. The is a switch on the back that would have to be in "LOW" impedance mode.
At 87db, those aren't the most efficient speakers. I don't know how loud you need to get. I don't know what the price for a pair of them in Japan is. In the US, you could do better with a pair of Klipsch RB-61-II.
Thanks for the suggestions. I've considered in wall speakers but the walls have very little space and they're a real commitment so I'm still too skeptical towards it.
It's a small room (5x5m) so I figure four large ones should be plenty. Especially paired with a sub.
The Klipsch speakers seem good but I'm not sure how I'd mount them.
Rather than the in-wall speakers that show up on the link from @americanaudiophile, scroll UP the page a little. Think about the indoor/outdoor speakers. They are going to be VERY durable and probably come with mounting brackets. Klipsch has similiar products, like the AW-525 or AW-600.