differences between various Muscat wines from Rutherglen, Muscat of Alexandria, Muscat de Saint-Jean-de-Minervois and Muscat of Rivesaltes.
John - do you want to know taste differences or production differences?
As you know there are many grapes called muscat, most of them in the same family but some probably not.
You've mixed apples and oranges, to use a clumsy cliche - i.e. grapes and regions. I'll tell you what little I know, but keep in mind that identiying these grapes is not always exact and in some regions, what was thought to be one thing may turn out to be something else. So my comments are general and there are probably a lot of exceptions.
Muscat Alexandria is a type of muscat grape, used in the others you listed. It is thought to be the oldest of the muscats and may be the ancestor of many. It is generally the grape called moscatel in Spain and Portugal, where it is often made into a late-harvest wine and in very minor quantities, into sherry. The sherries, at their best, have notes of orange rind and flowers, but they're also made in an oloroso style so they exhibit notes of roasted nuts and butterscotch too. The apparent sweetness depends on the producer, but they're generally not overly cloying.
The late harvest moscatel may or may not be fortified, but when it isn't it's really good, with the wonderful floral perfume of muscat and the concentration brought on by late harvesting. I actually like these wines a lot - they're relatively inexpensive for dessert wines and always pleasant.
Just as an aside, Muscat Alexandria is also the grape sometimes identified as Torrontes in Argentina, although that grape is also sometimes real Torrontes and sometimes a cross of Muscat Alexandria and something else. Again, accurate records were not kept and mistakes were made, sometimes innocently, so there is a hodge podge of grapes in South America. As an aside, I think that's fine - the key isn't the academics, it's the wine.
Muscat Alexandria is larger-berried grape than the Muscat de Petit Grains, hence the name of the latter; both are used in Rivesaltes. But that area is really confusing. There is Muscat de Rivesaltes and there is also just plain Rivesaltes. Both make sweet wine from muscat, although in the plain Rivesaltes I think they can use other grapes like grenache blanc. They are fortified wines, although more like port because there is a large amount of unfermented must when the fortification takes place. They sometimes taste a little like a cross between a tawny port and a sweet sherry, possibly because some people make it in a style that tastes oxidized. At least the few I've had seem that way. On the other hand, I've been told that the Muscat de Rivesaltes are not supposed to be oxidized whereas the plain Rivesaltes can be and often are, so maybe I'm confused here, although the nomenclature sure doesn't help. There are requirements for both the alcohol and sugar levels. Don't know them off the top.
These wines can be really interesting but it's hard to make any generalization about them, at least for me. Sometimes the producer will fortify the wine and then leave it to keep macerating on the skins of the grapes. They're picked fairly dried anyhow, having partly raisined on the vines, so the results can be interesting. In any case, the floral aromatics make the wines easily identifiable as muscat.
The Rutherglen is popular and much loved by some. Some producers, like Chambers, use a blend of the Petit Grains and Alexandria like they do in Muscat de Rivesaltes. Those grapes are raisined on the vine, fermented, and then as when the sugars drop to a certain point, fortified. Some are gradually fortified to attenuate the fermentation, some all at once. Some are also barrel aged for years, much like tawny ports, usually in used oak. They are much loved by some people but for me are generally too syrupy. I understand that there may be complextities to it, just as honey has different flavors and complexities, but that stuff is like PX Sherry to me - the sweetness often overpowers anything interesting that's going on. Again, just my 2 cents.
For my money, I tend to prefer non-fortified dessert wines, first because I just don't like to drink a lot of alcohol - I'd prefer to stay in control, and second because I think the alcohol masks the flavors more than it enhances them.
In addition, there are many other muscats made elsewhere - Austria and Hungary for example, sometimes use various muscats in their sweet wines, and Italy makes some wonderful sweet muscats as well.
Anyhow, hope this helps.