I think a weretiger is easy +10 ECL. Playtesting may show that to be too low, though. Compare it to an ogre at ECL +8 (from the Dragon article), and it is considerably better. More HD, much better stats, damage reduction, shapechanging, powerful natural attacks, scent, increased speed, etc. There were a couple long threads a while back that demonstrated that the ogre is balanced at ECL +8, so clearly the weretiger should be higher. The more I think about it, Mangrum's +12 is probably appropriate. Of course, this is assuming the player can control the lycanthropy. If not, it's probably not a suitable PC race, but who knows.
cthuluftaghn- If the player wants a werekitty and you don't want it to break the campaign, you might check out Arthaus' Denizens of Darkness, which includes templates for werejaguars and wereleopards. Both are still quite powerful, but they're closer to being in line with the power level you're for. Of the two, I think the leopard makes a much better PC race as it has a hybrid form, but as such it's probably too powerful to be in the ECL +4/+5 range that would fit for you.
cthuluftaghn- If the player wants a werekitty and you don't want it to break the campaign, you might check out Arthaus' Denizens of Darkness, which includes templates for werejaguars and wereleopards. Both are still quite powerful, but they're closer to being in line with the power level you're for. Of the two, I think the leopard makes a much better PC race as it has a hybrid form, but as such it's probably too powerful to be in the ECL +4/+5 range that would fit for you.