How to amplify 4 Main/Front Speakers

I unhooked the 8 ohm 303s from the amp and am just listening to the 640s and WOW do they sound better. I have now become a stereophile, formally, officially. The crazy thins is that I've had the 640s since about 1994 and never fully appreciated the sound they produce. From the things I've learned on this forum and elsewhere about making sure they're positioned correctly and far enough from the wall, I've re-discovered my own speakers. Thanks, all!

If you have the GFS-6, then there won't be a problem. Just switch back and forth between whatever speakers you want to listen to.

I use an Adcom GFS-3 on my second system. I usually select either my main, or deck speakers. You can play multiples at once. I do this from time to time when my house and yard are full of guests. Works fine.

I grabbed an Adcom GFS-6 6 Way Speaker Selector off eBay on the advice of a couple other posters as I want to send music to a couple different rooms but not likely at the same time. This unit allows me to select speakers in different rooms. There's not a separate volume control for each pair but that's not critical as I'll only be listening in one room at a time and will adjust the volume for the speakers I'll be listening to.

You can do it like Mechans says but he's right in that you may be pushing your Sony harder than it was designed to. If you end up trying it, I would suggest you start with very low volumes at first and if something doesn't sound right, shut it off right away.

Another factor that may be very important is volume. You will have no way of controlling volume independently for each pair of speakers. Assuming the amp can handle both speakers, one speaker may be more efficient than the other. Basically, one pair would play loud and the other would not be. If you keep increasing the volume, you risk damaging the speakers.

There are a couple of ways to do this. The two that I know of are called parallel and series connections.
The easiest is the parallel where you simply attach both pairs to the main speaker posts, that is the main and the smaller ones to the same post on each side. Doing this cuts the output impedance by half and may not be the best choice if your amp is not rated to handle this. The only amps that I would exclude immediately are tube amps which need all the output impedance they can get, and certain SS amps that warn against this. I had a Marantz power amp that specifically warned against it. The other less "amp taxing" method is called a series connection, which has one speaker connected in a chain with the next speaker in series. I don't remember the exact connections so do a search using that term and I am sure you'll find it.