Council of Elrond

Andúril, Flame of the West wrote: MP: 1(4) ...
In addition to Narsilʹs effects, Andúril gives its bearer 4 marshalling points...

To clarify, are these 4 marshalling points with Narsil in addition to the original 1 MP, or a replacement?

From where a concept that Anduril replaces anything?

Precedent set by the Dwarven Rings for example:
"Values in parentheses and brackets apply to a Dwarf bearer."

From which I interpret the +2(+4) to mean that Dwarfs get +4 prowess, not +6. This is also the way that I interpret other prowess modifiers like Orcrist, including permanent events like Barrow-blade.

OK.
I misread your question; I thought that it comes to replacing Narsil's MP (which is 3, not 1, shame ).
I think that values in parentheses or brackets aplly in some conditions, described on a card, and values outside (not always present) apply otherwise.

And there is inconsistency. Sometimes what is written in text (of Orcrist, for example) is not reflected in values printed in corners (only one is given).

Barrow-blade is other story. Unlike text of Anduril, the text of the card indicates thet it does not give itself a bonus to prowess. It increases the bonus that target Dagger of Westernesse gives without the card. However values printed in corner indicate that they work in the same way as bonus to prowess from Anduril.