for 50Mbps @25fps, 1920x1080 or 1280x720 are the only supported frame sizes. Is there a reason you need XDCAM? If so, then you can fit your video to the desired dimensions inside a 1920x1080 frame, but I think there's a missing piece or two of information you need to figure out why the client is requesting this format.
Is it going to be broadcast? Does the broadcaster have a more complete set of specs? Does the client understand the broadcast format requirement or do you have two competing sets of requests—the required broadcast format and the format you created your project in?
Premiere Pro can output to any aspect ratio you want without the need to make several different sequences. The video link below demonstrates this at about one and a half minutes in. The dimensions of 9X16 and 4X5 sound odd. 4:3 and 16:9 are the industry standards.