But do any of the lower sequence values have any significance in the RBF transaction?
Lower nSequence values imply a relative timelock, as specified by BIP68.Presumably, transaction replacement is governed by the transaction fee, so that one with a higher fee (as well as paying additional tx size * minrelayfee) replaces a lower one. But does the sequence number have any effect at all?
In terms of RBF, no. The nSequence number signals replaceability, but higher nSequence numbers are not prioritized over lower ones. It is based on fee alone.Meaning is an nSequence of 0 equivalent to FFFF 0000 and 0000 4000 and FFFF FFFD in this context?
Strictly in terms of the RBF, it makes no difference. But it could make a difference if your replacement transaction includes a timelock which means it can not yet be mined. 