That's definitely correct, $4 billion is meaningless. If it were $40b to corner 5G, Verizon and AT&T could come up with it and T-Mobile couldn't.
For significant business purposes, both Verizon and AT&T could borrow $40b easily. Whether for an acquisition or critical spectrum buy. They both have plenty of borrowing capacity despite being highly leveraged (it'll very likely end badly one day in the future).