Speaking of differences.. Here are the differences between the Jap and US games (Copied from GameFaq.com):
"Though the North American version of Super Castlevania 4 still has a
place in my heart for being the first one I played to death, the
Japanese version is so much better. Sure, they have the EXACT same
gameplay, but the North American version is heavily censored, and
there are some differences that got changed for some reason. The only
Japanese you have to worry about it is the title pic and the opening
text. Menus, and text everywhere else is English. Here are the
differences and advantages of the Japanese version:
First, the whip actually sounds like a chain, compared to a squishy
sound. Don't know why they changed this.
The pale, generic SNES lettering is replaced with a rich, stylized
font. Makes the North American version version look bland.
There's blood. Not bucketloads, but little touches here and there.
Blood on the tips of spikes for example. Also, on the title screen,
blood drips from the title pic. The huge difference is that the blood
pools, drops, and waterfalls in stage 8 are slime in North America.
It still looks cool, but blood fits the torture chamber atmosphere
better.
In the North American version, besides the Crucifix and Boomerang
items, all crosses have been taken out. Dracula's tombstone, the
casket enemies, caskets in stage 8, tombstones in stage 2, and more,
now have crosses. Also, for somereason Dracula's name on his tombstone
was taken out of the North American version as well.
The bosses are SLIGHTLY faster and SLIGHTLY more aggressive. You'll
notice Rowdain jump down from above more often, and Puxewil will use
his tongue, shoot fireballs, and move faster, for instance.
The statues in level 6 are naked in the Japanese version. In the North
American version, they're wearing robes.
There's no slowdown in the Japanese version! WTF? I can't understand
it..."
As for the Story intro.. I have no idea.. Since I can't read Japanese, and I can't find anything about it (read: don't have the patience to search any more)