Mixing video formats in same sequence

I'm somewhat of a beginner in Premiere Pro.  Have made several videos, know the basics of sequences and editing.  But to now, I've always used hi def (4K) video.  Now, I'm trying to make a sequence which mixes new video (3840 x 2160) with converted Hi-8 tape, at 720 x 480.  My sequence starts with the 4K video.  When I try to drag a lower definition video onto the timeline, it won't let me (the circle/slash symbol).  It *does* let me drag the audio onto the sequence.

How can I mix old and new video?  Surely this is something that is commonly done.  BTW, the frame rates are slightly different as well - 23.976 for the 4K stuff, 29.97 for the older video.