Disagree. Three is plenty (and I would assume perhaps Whitney can play some SG in a pinch).
Our 2012 title team really only had 2 "guards". Teague and Lamb.
That team only went 7 deep total. Miller played 26 mpg and Wiltjer 11. The 8th guy was Vargas at 6 mpg. You don't need to be 10 deep, and you don't need any more than 3 guards.
And to add on to that post. In 14 the Harrison twins were the only 2 guards. And we only went 8 deep total. Poythress 18 mpg, Dakari 14, and Lee 6 off the bench.
The '16 team had 4 guards. Ulis, Murray, Hawkins, and Briscoe, but Briscoe played SF and Hawkins played 9 minutes. Murray and Ulis played 35 and 37 mpg respectively.
In '17 we had 4 guards. Fox, Monk, Hawkins and Briscoe. Again Briscoe played SF. Hawkins did help out with 19 minutes. Fox and Monk played about 30 mpg.
You don't need 4 guards. Injury is about the only reason you need 4, but if you end up having to rely on your 4th best guard for a lot of minutes you are in trouble anyway, and expecting to have a 4th best guard good enough to still win a title with if you had injuries is pie in the sky dreaming.
I expect Hagans to play between 30-35 mpg. Expect Maxey and Quick to be in the 25-30 range. In fact I would argue trying to work a 4th guy in and taking minutes from those 3 is detrimental on the whole. At most you would need a 4th guy for is for very spot minutes in the case of foul trouble (like 5-10 or so in a random game or two). Getting a 4th guard isn't likely to have any impact on the team's outcome. Three is plenty.