thx for your Comment on my build, now the answers :
i picked it at lvl 4 because as a support who has the duty to heal heroes and tower, you have not the farm and maybe not the level for your ultimate must protect yout carry, at this level the carries are usualy in the most danger so you can suuport him and stealth him whil he /she is at flee.
my build is not the Final version, i think a Euls scepter would be a good idea, but as tank/mage you are not in the disabler role. you are a support that evade the damage from your team and heals a lot.