My idea is that each student would create their own vocabulary portfolio using the "vocabulary four square" that I brought to class last week. They would collect words that are difficult for them or words that they had learnt at home, on holidays or anywhere else (It does not have to be at school- It can be a word from TV series, a PC game, a book,...). Every week/ lesson a few students could share one word from their portfolio (when they think it could be useful for others) with their classmates and teach them/ present the word- show them the vocabulary four square for this word, tell them where he had found/ learnt the world and so on.
I think that it could be very useful to use this type of vocabulary portfolio, because there are 4 different "activities" which are connected with a new word. It is also a nice idea to let our Ss to share one word (four square) with their classmates and teach them the word (maybe it is something like peer-teaching? :)).
I liked a lot the Vocabulary Four Square last week, I think it is a good idea to work with new vocabulary through playful and entertaining way for Ss. I also appreciate the motivation part - "Every week/ lesson a few students could share one word from their portfolio..." and the fact that they can independently learn new words and show them to their classmates, which could via the peer-teaching (as Marťa wrote) extend vocabulary of the whole class.
I think the possibility to share the most useful word with their classmates could motivate the Ss to look for as many words as possible so they would find the best one. I can even imagine putting the best words somewhere, where everyone can see them and working with them even later.
The vocabulary four square is great because it offers us many ways to remember new words.
I had never seen the Four Square before and I am glad that you presented it during your lesson. Everyone is different and as teachers, we should try to show Ss different methods and ways to learn. The Four Square can be printed and put on the wall of classroom, so Ss can contribute during the school year.