Still, understanding the PMMC is an important and difficult question in the critical theory of modern capitalism and Woj is right, if that is what he means, that Marx offers little guidance here.
Unfortunately, neither Woj nor Marvin do a lot better. Woj talks about "effective control," but offers no metric for this. What is "effective" control, and how much do you need? He also offers no theory of the dynamics of societies where the effective control of social resources (of what sort?) is disproportionately in the hands of minorities. Such societies include all class societies, so an "effective control" theory runs together Homeric Greece, the Hanseatic States, communist Hungary, modern Holland, and the contemporary US. That's not to useful -- it suffers from the same problems as talking of "the rich."
[WS:] I am highly skeptical of grand theories that subsume complex reality into a few elegant equations. I'd much prefer a case by case approach in which a few general principles are applied to the specifics of individual circumstances. I am surprised that you have a problem with that approach, being a jurist in the common law tradition.
Specifically, class stratification depends on the specific institutional arrangements in any given society, the amount of power a group can wield in those arrangements, and the means through which they wield their power. In once society, that means can be property relations (cf. 19th century England), but in some other society, it can be control of the state bureaucracy (cf. the USSR or perhaps Japan or South Korea). In both cases you have highly stratified societies with relatively narrow groups wielding considerable power over the rest of society. Yet, if you concentrated on ownership alone, you would not be able to analyze the bureaucratic control, and even incorrectly concluded that such bureaucratic societies were free of the outcomes of the property relations (e.g. exploitation.)
So if you use two general principles (i) The devil is in specific institutional arrangements and (ii) it is the effective control of the institutions that matter, and perhaps add a test rule for the effective control - e.g. the ability to overcome opposition coming from other social or institutional actors, you can get some good mileage of that conceptual apparatus, in my opinion. Surely, it will not produce a grand theory of history and the world - as some Marxist and neo-classical accounts attempted to do. At best, it will produce a bunch of medium range theories with limited scope conditions - but I do not see anything wrong with that. As I said before, I tend to distrust grand theories - they may be elegant, but their predictive power is generally close to nil.
PS. As a matter of clarification, small businesses meet all four conditions that you mention: they own their assets, they can hire wage labor (albeit on a small scale), and produce goods or services sold on the market.
Wojtek