Oxford Dictionaries define a portmanteau as
A word blending the sounds and combining the meanings of two others, for example motel or brunch.
I've also heard the term apply to three- and four-word combinations, but I'm not sure if it's correct. For my blog, I wanted to describe the Latin word obstaculum, which is a combination of three Latin terms, and while I could use combination or portmanteau without anyone noticing, I began wondering if there are any terms that specifically describe three- or four- word mergers (in that the word is exclusive to the number). I also wanted to know if a portmanteau can correctly be applied if used for more than two words.