I have to say I would not bother with 4" speakers for the front or rear, even with aftermarket speakers the sound will never be really decent. I would go with 6.5" speakers in the front, and rear doors, an example of this is seen in the photos below of my '92 245 (taken when the car had 43k miles on it, currently 54k). I've done this to several 245s that I've owned, and so have many others that I know of. The fronts are pretty simple to install, very little modification required to do this. The rear doors don't have much behind the door panel to support the speaker so I usually take some aluminum sheet metal and cut some plates to hold them and pop rivet those to the door.
The front doors can normally take up to 1 7/8" mounting depth, possibly more, but there you may need to make some measurements with the window in the up and down position to be sure. There are other ways of mounting them, but I've found the easiest way is to support them on studs, I use 4mm screws:
http://www.threefattigers.com/Protocore/Volvo/SpeakerStud.jpg
If you pull off the door panel you will find four small holes surrounding the main speaker opening in the sheet metal. With manual windup windows, these 4 holes line up exactly in the ideal position for 6.5" speakers, however for the armrest used with power windows, you would need to use three studs in an triangular arrangement where you use the two existing rear holes and drill a single new front hole, and here you will likely also need to cut away a small part of the sheet metal on the forward side of the opening to accommodate the speaker basket/magnet, something like 1/4 inch; you may also have to enlarge the opening in the door panel for the same reason. The studs are arranged where the screw head faces the outside of the car, then one nut & inner tooth type lock washer securing the screw to the door, then passing through the door panel, then one nut+lock washer to create a back stop/rest for the speaker, then then one nut+lock washer to secure the speaker to the stud. These are installed after the door panel is in place. Finally they excess part of the screw is cut off using a dremel with a heavy duty cut off wheel. Finally, using some vacuum hose, with the ends cut at an bevel, to fill the obvious gap where the speaker bridges that central recessed area of the door panel.
Then I would toss in an 8" powered bazooka behind the driver's seat, as seen in the photos below. The carpet it's wrapped in serves to hide it, and to keep it from moving around and to protect the carpet below from wear marks. Amazing bass, can crank it way up, but outside the car you can not hear it at all, car is no louder than if all you had was a stock stereo. But open a window or a door and you can hear it inside your home. The controls on the back can be reached while driving, with your left hand. Can also offer a nice back massage when desired.
If I were buying components today for this setup, for the front I would buy either this:
http://www.crutchfield.com/p_130TSG....html?showAll=N&tp=105&tab=features_and_specs
or this:
http://www.crutchfield.com/p_130TSA....html?showAll=N&tp=105&tab=features_and_specs
most likely the second one.
And this for the rear doors:
http://www.crutchfield.com/p_130TSA....html?showAll=N&tp=105&tab=features_and_specs
Then the sub:
http://www.crutchfield.com/p_204TA850C/Bazooka-TA850C.html?tp=114
Don't know what to recommend in a 4" speaker, but then I wouldn't recommend them at all.
Photos:
Bazooka:
Only photo I have of 6.5" in my '84 245:
