It is highly unlikely that he'll be able to return to anywhere near the level he once was, at least not on a consistent basis. It is quite doubtful that he will hold up to the rigors that the rotational stresses of the sport requires.
As someone who works in the back care field one or two surgeries leave the lumbar spine compromised and structurally deficient let alone four surgeries. Let alone a fusion surgery where one can only hope all the hardware holds up under normal life circumstances, forget a professional golfer.
Lumbar fusion outcomes in Washington State workers' compensation. - PubMed - NCBI
Failed back surgery syndrome: current perspectives
He will come no where near reaching what Jack achieved and it's quite likely he'll have issues the rest of his life. It's too bad and has been sad to see. Here's hoping he can get a good stretch for a few weeks here and there and win a couple more, anything else would be near miraculous.