Additionally and furthermore are synonyms that mean 'also'. However, "moreover" tends to contain a different kind of information from what has come before. "Furthermore" simply signals that one is about to say something that will give additional support for one’s argument. "In addition" also has a closer link to the verb 'to add', so if you want the idea of 'adding', then use 'in addition'. Both words have the same meaning and are both used to the same degree in formal writing. ^{wordsmyth.net 1 2 3 4}