Contributions to 401(k) can only be made by employer payroll and cannot be backdated. The only contributions that will count for 2021 are contributions from paychecks that you receive in 2021 where the funds are actually deposited with the trustee in 2021. If you can afford to have $500 per paycheck deducted, that would be $4000 for 2021. If you could afford to have $2000 per paycheck deducted, that would be $16,000 for 2021, and would still be within the contribution limit. You could reset your deductions beginning January 1 to a figure that would be appropriate for an entire year.
It is very important to understand that although a 401(k) and an IRA have similar goals, they are controlled by different sections of the tax law and have some rules that are very different. You will want to familiarize yourself with both sets of rules. For one thing, the $19,500 limit on 401(k) contributions and the $6000 limit on IRA contributions are completely separate. In some circumstances, depending on your income level and financial resources, it may be possible to contribute $19,500 to the 401(k) and $6000 to the IRA, for a total contribution to tax deferred retirement plans of $25,500.
Your ability to make tax deductible contributions to an IRA may be limited depending on your income level if also participate in an employer plan, as mentioned above.
