The way merging works in this case is the following:
Merging the 5 * M Marth onto the 4 * one will give sp, the skills that the 5 * one had (which is only Falchion), but no bonus level.
Merging the 4 * M Marth onto the 5 * one will do the exact same. You'll have all those skills immediately available and already learned.
Promoting the 4 * into 5 * and merging will do the same BUT gives a bonus level, so promoting and merging is the beneficial, but also the most costly.
Hopefully this sloppy explanation helps.
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