Ok, it isn't the mass that is surprising, it is the volume. Larger (in mass) exoplanets have been found, sometimes they fall in to the category of Brown Dwarfs. But TrES-4 is hardly massive. According to the article, the density is .2 g/mL and the volume is 1.7 times that of Jupiter. That gives a mass of 1.7*(1.43128*10^15 km^3) * .2 g/mL = 4.866352 * 10^26 kg. Jupiters mass is 1.8986*10^27 kg. That means TrES-4's mass is only about one quarter the mass of Jupiter ((4.866352 * 10^26 kg)/ (1.8986*10^27 kg)= 0.256312651)