You don't need to trade to get your first 4*. Just do the mina's boot camp and helmut's boot camp. By the time you finish those two, you'll have at least 2~3x 4* and few 3*s as well.
People don't really trade 4*s for luna, though, rarely do exist. Main forms of currency used is time elixir and potions (tradeable, not the p). Just make sure you save them if u get them.