I haven't sterilized anything thicker than 8/4 and 24 hours is more than enough to reach 135F and stay there for at least 3 hours. I'm already starting at 110F or higher at the end of the drying cycle. For something 4" thick I'd have to run an experiment, as you suggest, but it's just a matter of time and likely not all that much as the piece you specified is only 4 x 4. It's just not that hard, or expensive, to sterilize (kill bugs) wood if you have a need.
John