Four courses is typical to get 15-16 units (most courses are 4 units, some are 3, and a few are other values like 1, 2, 5, or 6 units). Units nominally correspond to workload (1 units nominally means 3 hours per week of work), but actual workload per unit varies from course to course. Generally, courses with labs, art studio, music performance, large term projects, or computer programming tend to be higher workload per unit; it may not be a good idea to take more than two such courses together.