Berkshire Hathaway has $84 Billion dollars in cash.
Market Caps for airlines:
AA $24Bil
DL $37Bil
UA $24Bil
WN $35Bil
They could offer double the current market price for any of the Big 4 and still have Billions left over without having to take on any financing debt to make the purchase.
If they were willing to take on a small amount of debt, then Berkshire could buy all 4 of them.