It explicitly required 4 engines.
I can confirm. I was the Engineering Flight Commander for Speckled Trout when we signed the agreement with a certain organization to be the test bed for many of the new AF1 technologies, which meant I had to sign an NDA for competition sensitive stuff which got me a look at the early requirements pre-RFI so I knew what I could and couldn't say.
Been watching this for a while, knew about it for several weeks as one of my colleagues comes from the aircraft overhaul world in San Antonio and mentioned it last month. From what I gather, the backup effort goes back before the beginning of this year in the last admin. There just aren't, anymore, that many green -8 passenger versions setting around looking for new homes. ISTR only 2 VVIP versions were looking for new homes the last 5 years, the one with 16 hours that got parted out and the Qatari bird, which was the backup to the backup. Made for some interesting conversation because said colleague worked with Kelly Ortberg in a previous life.
A few other thoughts. If this is an interim meant to apply pressure to Boeing and manage risk on the two A's, then the full set of VC-25B requirements don't have to be in play. Namely, just do the coms, EMP, defensive suit, interior and leave off the AR. All the AF1's up to the current two did fine without it and 95% of the VC-25A missions don't need it. The folks who are supposedly doing it should be capable of that (see first para). When I worked at Boeing, I was actually a finalist for two jobs on the program and the PIM who sat next to me still helped on the program in his spare time, I'm somewhat familiar...
Also, FWIW I've been on the current jet, got the full tour, even sat in the president's chair in his office. Have the signed picture and presidential M&M's on display in my office. Gave the rest of the press kit to my parents since I missed my mom's birthday to work weekend duty, which led to the tour. Finally, funny story. Since I was the weekend maintenance duty officer, like any good officer when I saw the VC-25 on approach, I drove over to TA to get a look at the bird. The head of the SP's came over to ask me what I was doing, which I explained (I'm the MXG/CC's representative on the flightline this weekend, he wants a report of any significant events), so she then saluted me. The head of the aircraft security detail saw her salute me, so he walked over and asked if it was OK to park the C-32 next to the VC-25, since the VP and Sec State were in Crawford that weekend with W.