Some recent horses who set or approached monster speed figures at age 4:
Ghostzapper, Formal Gold, Candy Ride, Mineshaft, Congaree, Mazel Trick, Commentator, Lava Man, Lawyer Ron, Midnight Lute, Groovy ran his 132 and 133 Beyers at age 4.
The fastest figure horses in all of history -- Gun Bow, Discovery, Spectacular Bid, Coaltown etc. did by far their best damage at age 4.
Harbinger ran the fastest Turf race I've ever encountered at age 4:
Mossflower is a forgotten horse who also freaked at age 4 and went to the sidelines:
The good filly Mariah's Storm -- dam of Giant's Causeway -- ran a 120 Beyer at age 4. Inside Information freaked at age 4 and won the Breeders Cup Distaff by a pole over a great group.
Lady's Secret obviously freaked in a big way at age 4.
Phar Lap was invincible at age 4 -- winning four different Group 1 races in a span of 7 days including the Melbourne Cup under 138lbs giving big weight all-around.