As far as having Tefilla Betzibur with up to 4 people that already prayed, I believe that the Chazon Ish holds that this can only be done for shacharis and Mincha when there is a repetition of the Amida, but not during Maariv. How do most Ashkenazim posken in this matter?
There is a more general question as to whether a Minyan for Shmone Esrai comprising ten men some of whom have already davened is considered a full "Tefilla Betzibur" or whether it just permits the participants to say "Devarim Shebikdusha"- Kaddish Kedusha etc. There is a difference of halachic opinion on this question.Therefore, ideally, it is advisable, if possible to wait for a Minyan of ten men who have not yet davened. This is true of all Tefillot including Shacharit and Mincha. Orchot Rabbeinu Chazon Ish ch. 160 p.51 writes that in Shacharit and Mincha there is at least a "Tefilla Betzibur" in the repetition of the amida as opposed to Maariv where someone who davens with a Minyan of six and four is really missing out totally on "Tefilla Betzibur". Again- if it's possible, it's worthwhile to wait for a full Minyan; if not davening in this way at least permits one to hear Kadddish at the end of the Amida. The compendium of Halachot on Tefilla- Ishei Yisrael (34,9)- quotes the opinion of the Chazon Ish on the subject as "there are those who have written" since the distinction is not mentioned by most Poskim. In any event, the practical diference between Maariv and the other Amidot is minimal.