With four letters, you only need one forum, otherwise there will be more than one answer. Related words and related changes, such as +s +s, +ing, +ly, are listed as follows:

Focaccia vanilla olive oil bread (traditional Italian bread)

Fu Cachia si

focused

(Add in the comments below that Baidu is out of breath, but it can't be released ...)

Folic acid-folic acid

Folic acid

folic acid

Folate

Forum-Forum; Court (Roman) (plural of court)

Forage-cattle and horses eat, looking for food and grass (foraging)

look for

robber

Forwarder

forage grass

foraging

Foraminifera (a marine microorganism)

foraminifer

foraminifer

Foraminifera

Foraminifera

Foraminifera

foraminifer

foraminifer

Hole-a small hole, especially in the skull.

foraminifer

hole

perforate

perforate

Surprise, adventure, plunder

take by surprise

forayer

forayers

take by surprise

take by surprise