I went through this earlier in this thread, but I don't think $4.5M in cash is insignificant to Ohtani, especially in Oct/Nov last year, when the transfers were sent.
Between baseball (~$40M in MLB salaries) and endorsements he may have made $100M in his career thru 2023. At his income level in CA, half of it is going to fed/state taxes, and Japan tax levels are typically higher than US ones. He's paid an agent, has a team of advisors that likely draw some salaries, and likely has bought real estate and potentially made other illiquid investments.
Given all that, he probably has (had?) no more than $30-40M in cash on hand, so $4.5M would be well over 10% of his liquid assets at the time. It's enough to be meaningful.
This is a very rough estimate, especially with complete guesswork on the endorsements, but not sure what other major sources of income he would have had.