Hi there~ I don't have any cheap 4* for selling right now but I will try to give you some tips regarding getting some good ones.
You will be able to find several people here who will sell 2 fresh 4* for 1 TE don't try getting them from players who sell them for 1 TE each (like me for instance XD)
You might want to look out for specific 4* which will most likely have good effects such as Slow, Poison and Knockback. Also make sure that not every unit has 30 Deploy Cost, since certain Missions or Events have a limit of Deploy Points which ends up with you having to wait.
My suggestions for a fresh player would most likely be
Angela (Poison, S Speed! but 0 against Air)
Alex (If you play male units only)
Sergei (Knockback with decent stats)
Chris (No effect but good stats and gets bonus from speed demon skill I think...)
Estlin (Slow, really cheap deploy cost 10)
Miriam (Knockback, Really really strong!)
These cards are all from the Normal Card Set and people will sell them cheap, just ask them ^^ good luck and enjoy the game