Hello, Unless the 401k plans are with a former employer, I do not know how you have managed to default on your 401k loans as the payments are usually deducted from your paycheck.
However, if you default on a 401k loan with a former employer, the 401k loan will be treated as a withdrawal. You will receive a Form 1099R from the plan, and will be required to pay taxes on the amount in the current year and possibly incur a 10% penalty on the early withdrawal from a 401k.
Here is a resource that you may find helpful.
https://www.finra.org/investors/learn-to-invest/types-investments/retirement/401k-investing/401k-loans-hardship-withdrawals-and-other-important-considerations