Check out these rules on MLSnet. http://www.mlsnet.com/about/regulations/. These rules are theoretically current, but they appear to be from the 2001 season.
no, it has been 3 SI's at least since 2000. Wizards, Bunbury, Molnar, Johnston, the transitional International that year was Gary Glasgow.
MLS switched from 4 SI's to 3 SI's a few years back. http://www.soccertimes.com/wagman/2000/dec01.htm Of course, a lot of players who were once SI's now hold green cards, so my guess is that there're probably more non-U.S. citizens playing in MLS now than ever before.