?-4 is 500 xp for 40 stam or 12.5 ratio. Compared to other regular maps that's about the same. For comparison 5-4 is 12.79 and 10-3 is 13.7. Also I'm pretty sure ?-3 is like 13.3 ratio.
Its also leagues ahead of ?-3 which gives maybe 400 gacha if your lucky while ?-4 can give almost 900 a run.