Need help to figure out the hardware

I’m trying to convince my boss to switch to Asterisk, instead of a close solution (and expensive, 1 000 000$!)

I need to know what will we need to run about 4 000 concurrent calls?

Most of the agents will be on SIP phones, but all the lines come from PRIs.

Also, as a prove of function, it may help me to know if there is compagnies out there running Asterisk with more thant 4 000 agents on the lines.

Thanks a lot!