It's not gonna happen, guys.
I've said it pleanty of times before. Disney is absolutely horrible with releasing full season sets. We're lucky to have even gotten season 3 on DVD.
It's a lot tougher to get variety shows onto DVD because of all the red tape they have to go through to finalize the product. They have to make sure all of the master videotapes are there so that they could be digitally remastered. They need to also make sure they have paid all of the guest stars and/or estates royalties for using their likeness on the show. They need to pay additional royalties for any music and dance numbers used. The variety business can get pretty messy, which is why there are so many "Best of" DVDs of popular shows like Carol Burnett, Flip Wilson, and Ed Sullivan. Laugh-In FINALLY released the complete series onto DVD. After several Time Life infomercials of Greatest Hits DVDs only being released, they're finally releasing all of their episodes. Imagine the struggle it took for Time Life to get approval from everyone. It wasn't easy.
I'm not saying that Disney will NEVER release seasons 4 and 5. But it may take them a while. They need to get approval from everyone involved (or their estates if they're deceased) and the licensing holders to the song numbers so they can finally put the DVD out. We just need to be extra patient before that will ever come to fruition.