I have a slightly different view... would go LH>TK>UA, but that is dependent on the time of day.

I am a relatively frequent visitor to all three lounges and find the TK lounge crazy in the afternoon. Also, power ports and a place to work are important to me. The TK lounge has very limited places to plug in and no high tops to sit and work at like the LH and UA lounges.

On the food front, you can't go wrong with TK or LH, and both are better than anything UA has at IAD. I personally love Mediterranean so I'd go for the TK food.

Just my two cents.

Edit: realize you have 4 hours. In that case, go to both of them if you feel like getting a change of scenery. The IAD UA clubs are totally ok to skip IMO.