4 rounds, one round per day seems a little weak to me unless some of these rounds permit >18 holes. It's an interesting sample size debate though.
There is a point beyond which more rounds will not help determine who is better. At some point, the players that are clearly better are out of reach, and the positions of the players that are nearly equal in skill just switch back and forth randomly.
The Open Women experience this a lot. There just aren't enough entries to justify so many rounds for them.
We can also see this happening after 5 rounds for Open.
The effectiveness of a tournament can be measured by the Scoring Spread Width of Total Scores. For Open at 2015 Pro Worlds, this was 62.25 (with or without the Final 9).
If the Semi-Finals had not been played, the Scoring Spread Width of Total Scores would have been almost the same = 62.10. This indicates that the Semis just shuffled around the players of near-equal abilities (and, of course, it did nothing to help sort out all the players who didn't play the Semis).
In other words, 5 rounds of 18 is enough to sort players out as well as they can be sorted.
If only four rounds had been played, the Scoring Spread Width of Total Scores would have been smaller, around 50 depending on what course would have been skipped. This indicates the 5th round still contributed some information about the ranking of the entire field.
If the players don't feel that 4 rounds of 18 is "enough", a 4 round tournament of 21 holes each would produce about as much information as 5 or 6 rounds of 18.
However, I support trying 4 rounds of 18. Our primary goal is to produce excitement, not a clinical, perfect ranking of players by ability. The players know what is more fun. Perhaps they know instinctively that leaving some chance to get a better-than-you-deserve ranking will give the players a reason to think about taking more risk.