Bob,
Put another way, the famous Hogan 1 (or 2) iron shot at Merion occurred on a 458 yard hole. I presume his shots were about a 250 yard drive and a 210 1 iron. I think the prevailing (formerly conventional) wisdom is that a long par 4 ought to make you max out a driver and long iron.
To put todays average tour pros - but not necessarily the top driving length pros (as Hogan wasn't necessarily the longest hitter in his day, just the best player) in that position on a par 4, you would have to assume about a 290 yard drive, and 235 yard 2 iron (since 1 irons are so rare) for a total of about 525 yards as the maximum par 4 on the PGA Tour.
The last at Atlanta Athletic Club played as a 500 yard par 4, as I recall, and was difficult for some in the PGA field to reach, but of course, it was designed as a par 5 with a pond in front. I doubt I would design it that way if it was a par 4.
To recreate the Hogan/Merion challenge for the very longest hitters, which is perhaps an unfair standard, you might have to figure a 325 yard drive and up to a 250 yard 2 iron, (since 1 irons are rare, except for those that are actually called 2 irons!) for a whopping 575 yard hole. Perhaps we could round that down to 560, to make it an even 100 yards added in just over 50 years.
And, I just read this on MSN:
"Wie had a good chance to take the lead by herself after hitting a 6-iron to 6 feet on No. 18. She just missed the putt, though, and the first round closed without one player making birdie on the difficult, 459-yard par-4. The hole played at an average of 0.741 strokes above par."
So, the 460 yarders are a 6 iron, not a 1 iron, for the longest hitting women golfers now.
If the USGA ever changes the distance standards, I doubt they would go as high as necessary for the pro tour, since the pro tour is such a small slice of golfing life. However, I think 275, 500, and 725 yard limits splits for 3, 4 and 5 par holes is realistic right now, since the standard is based on scratch players and there are usually multiple tees for shorter players.
I think any new TPC Course might use 300, 525, and 750 as limits, and the option for a specific course or tournament to exceed a recommended distance for a par 4 has always existed.