Roughly 20% of the 4-letter words can be constructed by adding S to a 3-letter word. There's some rules that make remembering these easier.
The rule has three cases.
The following words ending in Y cannot take an S.
FEY GEY HEY
The following end in YS and are four-letter words.
ABYS DRYS WHYS
The following 29 are four-letter words.
AAHS BASS BOSS BUSS COSS DAHS DISS DOSS EDHS ETHS FEHS HAHS HEHS HISS ICHS KOSS LASS MASS MISS MOSS MUSS OOHS PASS PEHS PISS PUSS TASS UGHS WISS
The following 90 3-letter words cannot take an S.
AFF AFT AGO AHA APT ATT AVA AWA AZO BRR COR CUM DID DUI EAU ELF ERE FAR FER FEW FIE FOR FOU FRO GAN GOR GOT HAD HAO HEP HIC HID HIM HMM HUP ICK IFF JEU JUN KOI LED LEU LEV MED MEN MET MIM NAE NAM NAW NEE NOO NOR NOT OFT OHO ORA OVA OXO PER PHT PIU QUA RAN SAD SAE SAT SAU SEN SHA SYN TAE THE THO TOO UMM UPO VEG VIA WHA WHO WUD YAR YEP YET YOM YON YOU YUM ZOA