How many hours should lapse between work shifts? Is only getting 4 hours between shifts legal?

Unless you are working in a high risk job like pilot, truck driver, crane operator, etc = a company is not required to provide any time between shifts.

Haven't you ever heard of the phrase "pulling a double"? It means the person has two shifts back to back with no break.

Yes, it is legal in California if you are an adult. In California, only children are required to have breaks and time off.

Without a Union Contract or Personal Contract specifying the amount of break between work shifts, An adult can legally be worked 24 / 7 without breaks or time off as long as the employer is willing to pay for the hours you work. The employer can legally set work hours as the employer deems necessary. Your only option is to quit if you don't want to work the hours set by your employer.

California does not have a law limiting the hours an employee can be required to work. A 24/7 schedule is legal as long as the employee is given meal breaks and is paid overtime. The employer has a right to fire an employee that refuses to work requested overtime.

No legal requirement for time between shifts.

They call that a split shift and some restaurants and hotels do that.