Sometimes bigger is not better, requirements are to be seen.
If the ZDK-03 can do then job, then why need a much bigger platform to do the same job.
And 4 IL-78s are not gonna do the job, we need more platforms. Atleast 10-15 platforms would be needed to have a non stop AEW&C capability in case of an emergency.
Do you know how much it costs to acquire the Boeing platforms ??
Azad Bhai, its good to think with imagination with no boundaries, but in reality, a lot of factors decide about weapon systems and their procurement, like minimum requirement and specifications of the weapon system, operational and logistical issues, budget and then the maintenance issues in the long run.