Grey, I know that seizures on a two stroke with lean jetting can be likely, I have toasted my share of bikes. I also know that 4 strokes last a lot longer because the piston is always in its own oil bath. However, won't lean jetting on a four stroke cause unnecessary heat build up which can cause premature engine wear?
There are several things that make the 4-stroke a different animal than the pingers. For one, the lube oil in a 4-stroke is distributed the same at all throttle settings, and just by the nature of the lube system, there's a lot more of it where it's needed. For another, the piston and rings don't have to deal with running over a set of gaping holes at high speeds.
But as to the question of air/fuel ratio, they are different in that regard, too. The first point to remember is that a two-stroke gets almost no benefit from the fuel characteristic called heat of vaporization, which is a major factor in keeping a four-stoke combustion chamber at a reasonable temperature. As the fuel enters the chamber it is vaporized, and in so doing, absorbs heat from its surroundings. Even a very lean mixture will do this nearly as well as a rich one. This happens in a two-stroke, too but it happens more or less entirely in the crankcase, where it is much less important. This is one of the reasons that fuel needs to be blended differently for different applications. The vapor rates, etc., can be adjusted to address this to some extent.
In order to completely burn a pound of gasoline, and have no leftover fuel or oxygen, you need 14.7 pounds of air. That ratio, 14.7:1, is the "stoichiometric", or chemically balanced, rate at which fuel and air must be mixed in order to completely burn. Because a certain amount of fuel will never be burned in any given power stroke, the best power is usually produced in a 4-stroke at about 13.6:1. Modern cars run as lean as 16:1 and leaner under light loads.
When the mix is correct, combustion takes place, and there is "no" leftover fuel or air (ignoring the unburned fuel that results from various inefficiencies). If it's too lean, the one obvious problem from a performance standpoint is that you could have gotten more fuel in the engine, and thus, more power out of it. From a mechanical viewpoint, yes, the temperature may run a little higher, but it isn't a problem when using good fuel blend unless the cooling
system isn't able to deal with it, and these days, it usually is. As long as the ignition timing doesn't contribute to the onset of detonation (starts as "pinging", but gets much worse), there is no real worry in a 4-stroke, because the performance will deteriorate to the point of misfiring before it causes any damage.
Rich mixtures, OTOH, leave a lot of fuel leftovers simply because the available oxygen was completely used up before it all got burned. This extra fuel forms carbon deposits in the combustion chamber and on the exhaust
valves, contaminates the lubricating oil when it is forced past the rings, and in extreme cases, can wash out what little oil there really is on the rings and interfere with their ability to seal. The carbon deposits can become bad enough that they create "hot spots" that will become sources of pre-ignition, which will cause detonation. Carbon can also pack around the top ring and interfere with sealing.
From a performance standpoint, rich mixtures don't burn hot enough to generate good power, and, of course, if the mix is much too rich, the engine will blubber and miss.
If you were to look at the range of jets that would allow an engine to run without misfiring either from being too rich or lean, you would find that the ideal power mixture would fall nearer the lean limit than the rich (probably about 30-40% up from lean). It is this tolerance of being over rich that so often fools people into jetting richer than they should.