In order to successfully balance a family with household bills in The Sims 4, you'll need to find a fast and efficient means of earning an income. While there are several options (including working through one of the career tracks), you should quickly discover that the fastest way to make money in Sims 4 is to work from home. Careers can be interesting, but they may also be limiting.
Many players have found that the best way for a single Sim to make money in the game is to paint on an easel daily.
Players wishing to make money most efficiently should first consider their Sims personality. If you plan to utilize the painting option, it is best to give your sim traits which will support their efforts by allowing them to accumulate reward points (which can be spent on rewards which will allow the sim to spend more time painting). Painter sims should possess the traits "creative" and "art lover" at the minimum. "Cheerful" may also be useful as it causes the sim to roll a "happy" moodlet as a result of the trait.
It is also recommended to make your Painter Sim a Painter Extraordinaire aspiration, as this will allow you to accumulate reward points while painting. You'll only have to leave the house a few time to visit the museum.
When you're first starting out, purchase an easel for your sim (only the basic easel will be available at this point) and begin painting. In the beginning, you'll only have "classic" paintings available. That's okay; These generally sell higher than paintings of other types (except perhaps for abstract and occasionally a "realism" painting). If you're playing without cheats (and in particular if you're playing a Legacy Challenge), "small" paintings will earn you the largest return on investment. If you're hoping to make money in Sims 4, starting out with small paintings will earn you enough money to continually purchase new canvases to paint.
A small canvas costs 50 simoleans. Early on, you will earn anywhere between 6 and 10 simoleans per painting on top of earning back what you paid for the canvas. In other words, expect to "sell to collector" for between 56 and 60 simoleans per small painting.
If you're playing with cheats, are living on a smaller lot, or have been painting for a while, you will actually make a greater return on investment by purchasing large canvases and painting on those.
When the Pop Art paintings become available, do not take this option! Continue to paint classic paintings until you've reached a skill level which allows you to create high quality abstract paintings. This is where the best money is.
In order to make the most money painting in you need to make some other considerations. The first is how to most effectively improve your skill level in the shortest amount of time. Placing mood objects which give an "inspired" ambiance in the room will help to keep your sim in the right mood for increasing all creative skills (including painting). Mood objects vary, but most in the game come as the result of rewards. Plumbob lamps available for having purchased and registered The Sims 3 are often mood objects, and at least two provide an "inspired" moodlet. Look for the purple and the teal plumbob lamps.
To get the best reward objects, however, you're going to have to enter a creative career. Because your sim is already painting, the Painter career is your most obvious choice for this purpose. The personality traits that you chose (if you've followed recommendations thus far) will make your sim strong for the painter career, and he or she should move up through the ranks quickly. Doing so will provide objects which provide an "inspired" moodlet, but most importantly, it will earn you an easel upgrade.
Most importantly, the painter career affords the player the opportunity to purchase an improved easel. This easel will allow the painter sim to produce higher quality paintings, which will net more money. Be sure to enter the Master of the Real branch when the career track splits: This provides the best easel in the game.
Note that the painting career also allows the painter sim to sell his or her paintings "to gallery" instead of "to collector" for a higher price, but only after the sim has reached a particular skill level.
Painting is far from the only way to make money in the game, but it is the best way for a single sim to earn enough money to build a substantial home and family.