Reddit user TwistedMexi found hidden skill cheats for The Sims 4 shortly after launch. SimGoodie preserved the command format and the original skill identifier list.

Turn on testing cheats first:

TestingCheats true

Then use the skill-level command:

stats.set_skill_level <skilltype> <level>

Use a level from 1 to 10. The source note says the Major_ prefix is actually AdultMajor_, but the shorter identifiers below were the working format listed by the original page. Do not shorten them further.

Skill identifier Notes
Major_Fishing Source-listed skill type
Major_Mischief Source-listed skill type
Major_Guitar Source-listed skill type
Major_Reaping Source-listed skill type
Major_Programming Source-listed skill type
Major_Gardening Source-listed skill type
Major_GourmetCooking Source-listed skill type
Major_Comedy Source-listed skill type
Major_Charisma Source-listed skill type
Major_Writing Source-listed skill type
Major_VideoGaming Source-listed skill type
Major_Violin Source-listed skill type
Major_RocketScience Source-listed skill type
Major_Painting Source-listed skill type
Major_Piano Source-listed skill type
Major_Logic Source-listed skill type
Major_Handiness Source-listed skill type
Major_HomestyleCooking Source-listed skill type
Major_Bartending Source-listed skill type
Skill_Fitness Source-listed skill type
Skill_Child_Social Source-listed skill type
Skill_Child_Motor Source-listed skill type
Skill_Child_Creativity Source-listed skill type
Skill_Child_Mental Source-listed skill type

Example:

stats.set_skill_level Major_Gardening 10

The original article credited TwistedMexi's Reddit post as the source for the discovered commands.