効率が悪いですが、mciで同じ動画を複数登録してやるの方法がわかりやすいかと
dialog "", 16
if stat = 0 : end
videofile = refstr
mci "open \""+videofile+"\" alias video1 type mpegvideo style child parent "+hwnd
mci "open \""+videofile+"\" alias video2 type mpegvideo style child parent "+hwnd
mci "open \""+videofile+"\" alias video3 type mpegvideo style child parent "+hwnd
mci "open \""+videofile+"\" alias video4 type mpegvideo style child parent "+hwnd
mci "status video1 length"
time = stat
time1 = 0
time2 = time*1/4
time3 = time*2/4
time4 = time*3/4
mci "put video1 window at 0 0 320 180"
mci "play video1 from "+time1
mci "stop video1"
mci "put video2 window at 320 0 320 180"
mci "play video2 from "+time2
mci "stop video2"
mci "put video3 window at 0 180 320 180"
mci "play video3 from "+time3
mci "stop video3"
mci "put video4 window at 320 180 320 180"
mci "play video4 from "+time4
mci "stop video4"