You are not getting it. The text could basically read: "Deathrattle: If this minion has 4 or more attack resummon this minion".
Thus it summons the basic form of the minion. In this case Recurring Villian with a deathrattle as it's deathrattle minion pr default. The Undertakah is a battlecry minion pr default, and won't keep the deathrattle.
It makes sense. I played with a deck that was based around Recurring Villian, Zerek and Da Undertakah at the start of the expansion, and Da Undertakah was not a permanent thread but rather a: "Kill this dude and deal with a board of 8/5's".