Is It Possible For My Sim To Have Multiple Jobs?

You can't. Not too sure if you will retain the level if you go back. Maybe you could try on some random made up sim (wait for them to promote to 2nd level and then quit and go back) and see?

No, and he wont go back to the highest level of the culinary career, I think you'll be in the first level of the branch you chose

i don't know if it is possible to have two jobs at the same time but i do know its possible to quit your job and start a new without losing your current level (meaning know matter how many jobs you have and quit your level will stay when you re join) at least that what happen to me i leveled up in every career and before deciding to re join a previous career- sims 4

I know you can own a retail store and have a career, but you can't juggle both.

It would be hard to have two careers because the job times would overlap.

In a way you kind of could if you worked an actual job and then on the days off maybe do something such as paint and sell the paintings or write a book, etc. without picking up the actual careers that may go with said hobbies. That's just my thoughts on it. But two actual careers I know you definitely can't

I'd love if they could do this with part time jobs - have some that are one part of the day, some another, weekday ones and weekend ones. Then some could double with some others.

No, I'm afraid this has yet to make it into the game. If an extra income is desired/needed, there are a few things you could have your Sim do in their 'off' time. Gather the wild fruit/veggies and flowers around your lot and sell them from the inventory. Paint and sell the artwork; write and sell the books/articles. I'm currently playing a Legacy Challenge family, and money is really tight, so my Sims gather the wild fruit/veggies and flowers from around their lot. Comes in handy, and has actually helped them keep the lights on. o:)

Not unless the other job is unofficial self-employment.

Sadly it's not possible.

Having more than one job would definitely have those schedules clash together. You can kinda give yourself an at-home job I guess, like selling paintings and publishing books. Or you can use that cooking skill to have a bakery and get a little extra dough from there.