What if you did include X-cost cards along with the Princes, and just played the princes after you drew them? They aren't terrible on their own, and their effects can be strong when triggered. It never says that you straight-up can't include them in your deck - they just have to not be there when you actually play the Prince.
first prince of the 2/2 that gives everything in your deck +1/+1 sounded like it had potential, second prince of the 3/3 that transforms into a 3/3 copy of a minion sounds like it could have combo potential, this one... It's basically just crippling your deck for a single ok card. 4 mana 4/4 is already kinda cutting it for playability but with taunt and lifesteal if it was just a 4 mana 4/4 taunt lifesteal it would be fine.
Here's the other thing. Let's compare this to Wickerflame Burnbristle. like Prince Valanar the stats for 3 mana isn't good. actually worse but it has the added divine shield. Why couldn't Valanar have the divine shield? Sure it isn't a class legendary but you are also making us have a deck with no other or very few 4 drops. Just something more than just a 4 mana 4/4 taunt, lifesteal may allow it to be a little more playable.
The requirements are pretty big since the 4-mana slot is pretty crucial, and if you do actually meet the requirements the card is still incredibly average. Maybe if it would get Divine Shield as well this would actually be somewhat good.