Is it as simple as picking two cards out of the four that would make a decent Hold Em starting hand or is the playable range completely different?
I agree with Mr. Booth, this question is far too broad, more over, it is not simply answered in one compact round.
If you know anything about both games, you know intuitively that they dictate wildly divergent strategy. For the same intuitive thinking, you should know that just a cursorial perusal of 2+2 and Google will yield all the mathematical and theoretical data that purports the same.
Answer: It is completely different. Hand values preflop depreciate immensely overall as PLO is by nature an exponential-pot-increasing format; it is sometimes impossible to get a favorable stack-to-pot ratio in the middle before the flop is seen so as to pull old NL thinking into play, "Just jam any flop. The math/relative hand strength justifies it." Not the case in PLO, as the remaining rounds are rife with potential victories spoiling the bettor who is never required to showdown the winner. In that, an aggressor can win at anytime by exercising that aggression, entitling the reward of a showdown free victory, NL and PLO Omaha are both beautiful indeed. However you'll find that due to skewed maths, wider opening/limping/**ery ranges, there is a lot more room for maneuvering in PLO postflop V NL postflop.
I agree with Mew, you wanna look at the hand as multiple viable hold em starting hands, particularly you'd like to be dealt double suited connected cards, like AsTsKhQh, or double paired double suited such as AsAhKhKs.
Top pair, any two pair including top, is generally worthless in showdown as your opponents are quite likely to fill nearly any draw imaginable (and -unimaginable, to the seasoned NL player) frequently.