How can I use a .mp4 as a startup movie instead of .bk2?

In UE4.27 I can not use a .mp4 video anymore, only a bink video file. The RAD Video Tools give me only a bink 1 video file instead of 2.

I would stay far away from bink, as its compression ratios are unparalleled in how bad they are. Literally, they are worse than h.263, theora, and other very old codecs.

On my end, h.264 is working just fine in editor.