Which is best for FI-SP418 sealed or ported box? What's the best box size for both?

Not enough information to answer that.

Killer sub in either alignment, you just need to guve us more information.

are you looking for sq or spl?

Major thread jacking right here. Is that Fi SP4 a more sq based or spl sub? I know enclosure makes a difference but some subs "lean one way or another". I'm looking for a stupid loud sub that has good sound quality.

All depends on definition and the enclosure that is designed. You can turn anything into a fart cannon though, but not everything into something that sounds good.

Which is best for FI-SP418 sealed or ported box? What's the best box size for both?

Send me one and I'll do some testing for you

go ported !