Unfortunately you can only update Suites to Suites or standalone products to suites. There is no upgrade path for individual products within suites to standalones.
So you'll have to update to the next version of Master Collection.
Hold off on upgrading to CS4 though since CS5 will be announced on April 12.
Thanks for the information. When does CS5 come available for purchase?
Based on previous releases, usually within 4-6 weeks after the launch.
So CS5 will probably start shipping sometime in May.
There is usually a grace period set after the announcement to that if you purchase CS4 in that grace period you'll qualify for a free upgrade to CS5.
Another approach would be to download the CS4 Master Collection Trial, run it until it expires and purchase CS5 in the meantime.
Or you could just sit tight and wait until CS5 ships.