We've already seen huge ensemble casts in the MCU. The first Avengers was big at the time (though X-Men was probably bigger), and then Avengers: Age Of Ultron and Captain America: Civil War dialled things up massively with even more actors as Marvel refused to kill too many characters off. That was nothing compared to Infinity War.
The story as a whole has an astronomical number of key characters, and Kevin Feige has been very clear - not least to Chris Evans - that there is so much story and so many characters to balance that Infinity War has to spill into Avengers 4 (even if it isn't titled Infinity War Part II any more). And though they cut down the cast with that stunning ending, that doesn't mean we won't still see a huge number of characters in the sequel.
Still though, with the secrecy over what will happen in Avengers: Infinity War, we don't have firm confirmation that too many characters are back for its sequel.
There are some we know about though. Thanks to set photos, Infinity War's end, actor confirmations and the word of God (or Kevin Feige as he's more conventionally known) here's who we know for certain is involved so far in Avengers 4...