In fact BL-4C fits right in an Mp4 rockchip player.
But I have done the same that Seraphim making fit a BL-5C into my device by making it more thin.
It worth the effort. Nokia battery (in fact is a Samsung battery) last at least three times the original chinese battery.
Rockchip device demands more battery than an Ipod clone. Manufacturer put a special remark about this, how is to s switch off the device when it's not in use.
I find out that is a Samsung battery because in the process of make the BL-5C fit in my machine I remove Nokia cover, now is a silver battery with a small Samsung marked on it.
Not a Nokia anymore :P