It is not possible to buy a 4 trillion public company
Sun, Lucent, Yahoo all had massive valuations at their peak but eventually dwindled and got acquired.
It's always possible for a massively valued company to stumble, fall, and become a husk of what it once was. I don't think Google/Alphabet is immune to this even though their absurd cash cow from ads make it very unlikely at this exact moment.
Somehow they seem to have more despite not earning it for such a long time.