Standard appears to be available here:
Also matrix is here:
It will be different for your location, but if you click on the "purchasing options" from the Flash Builder 4 home page you should be able to get Standard.
I made a phone call to Adobe today, and it seems there is a problem with the store upgrade otpions.
I asked two quotations from their selling team so here is what I know :
FB3 PRO -> FB4 PREMIUM = 229€ ex VAT, 283.45€ inc VAT (boxed version)
FB3 STD -> FB4 PREMIUM = 369€ ex VAT, 450.89€ inc VAT (boxed version)
Call them to buy it properly, or wait until they fix the online store.
who seems to reply faster than big A...