An ERA of 4.00 is pretty decent, given that it's backed up by a WHIP of 1.14. Usually a WHIP that low indicates an ERA around 3.00, so it shows that Kim has probably had bad luck so far and his ERA will go down. Also, the league average for ERA is around 4.60 or something, so yes that is decent. Better than Greg Maddux or Derek Lowe's ERAs at any rate.
I don't think the DBacks can possibly continue to give such pathetic run support. After all, they have won their division the last two years, and that can't be entirely on the shoulders of RJ and Schilling, they have a decent offense and it will get rolling.