Summary:
Having used both the 3a and 5 models, I think the 4 really does fit between them. Compared to the Five, the Four has better center channel processing, which I check on all of these models with "Sgt. Pepper's Lonely Hearts Club Band", with its impossible left-right separation. The Five wavers between the center and the sides. The Four and 3a commit to sending a vocal or instrument to the center.
The Four is less of a hassle to use than the Five because it has front panel buttons for volume and mode.
The Four has the 5-sided steering indicator, but in fact the only mode with split surrounds is Rock. Its problem is that the left surround is pretty much following the left main, and same on the rights. The 3a's 70 MM Wide mode seems to combine elements of the old 70 MM and the new Rock mode, to great effect. The Five seems to have a more primitive Rock mode.
If you have a D/A convertor and you need an analog pre-amp for a volume control, the stereo bypass mode on the 4 and 3a are very welcome. Their music modes are fine but sometimes direct too much sound to the center. The Five is really at a disadvantage here, lacking both the bypass mode and the improved Rock modes.
The 70 MM mode on all of these is fine with antenna sources, but with laserdisc the edge goes to the 3a, with the 4 a hair behind.
The 3a has so many features that I can't begin to describe. However, if you're simply running a 5-channel system thru bypasses on a Dolby Digital processor, and one subwoofer, you probably can live without the 3a.
Right now the 5 runs about $100-150 used, the 4 is about $200, and the 3a is about $300-400. Value ratings are based on that scale, as something you just picked up for your remaining analog sources.

Strengths:
stereo bypass mode

Weaknesses:
No level meters

Similar Products Used:
Fosgate 3a, Fosgate 5