If a child received 5 dtap doses (instead of the recommended 4) before age 3, do they require a booster at age 4-6?

Booster at age 4-6: Booster at age 4-6 necessary even if 5 DTaP given before age 3.

Yes: You can consider the extra dose early just that, an extra dose. The development of immunity in the human system depends both on doses and the time interval for boosters. Depending on the age when given, some have a stronger response than others. Most infants have poor retained immunity for DTaP so it is given several times early on, with boosters beginning to spread out as they age.