If you killed the Ender Dragon and place down the end crystals before the dragon finishes its death animation, it will drop the xp and open up the exit portal.  However, the end crystals don't do anything and it will not resummon the Ender Dragon unless you place the 5th end crystal.  This applies to the 1st, 2nd, 3rd, etc. kills of the Ender Dragon.

Video Link: https://www.dropbox.com/s/zexqvil8l3tsw56/Minecraft_%201.16.5%20-%20Singleplayer.mp4?dl=0