Final answer:
The correct answer is option 4. Chinese Americans were not a significant part of the California population in the mid-1840s as their arrival in large numbers occurred later during the California Gold Rush.
Step-by-step explanation:
The group that was not a significant part of the California population in the mid-1840s was Chinese Americans. At that time, the main groups in California were Native Americans, who were original inhabitants of the region; Mexican Americans, who were part of the population due to California being a part of Mexico until the Mexican-American War; and European Americans, who were starting to migrate westward.
The Chinese Americans influx occurred mostly during and after the California Gold Rush, which began in 1848, reaching its peak in the 1850s. This was after the mid-1840s period in question. Therefore, Chinese Americans were not present in large numbers in California during the mid-1840s and did not contribute significantly to the population until later in the 19th century.