It seems Delco has discontinued the vacuum modulator for the 440t4 long time ago. Only aftermarket brands are available now. I wonder what brand is recommended?

About anything from a reputable manufacturer and auto parts store will work. I get mine from Napa for my 4T60 and THM400'* and have never had a problem. I don't currently have one (that I bought at Napa to replace failed factory units) that is less than 10 years old and they still work fine.

I never had an issue with the one from Napa in the years I had it on the Park Avenue. It also gives you the opportunity to use it to increase pressures a bit to keep heat down and firm up shifts.