Custom hp

What command are you using to summon the zombie?

/summon zombie ~ ~1 ~ {HandItems:[{id:"minecraft:iron_sword",Count:1b},{}],Attributes:[{Name:generic.maxHealth,Base:5000}],Health:5000.0f}

I reliazed that the max hp was about 1050

After summoning, use the following command and show us the output:

/entitydata @e[type=zombie,c=1] {}

and again after hitting it once

PS: put your commands between {code} blocks.