how do I record investment change in value

How are the investments in the 401(k) recorded in Quicken? Are there one or more securities (usually mutual funds) or is it something else?

Usually there would be contributions from you and your employer, which are used to buy shares of securities and their share prices change over time.

I'll echo the above posts. You simply haven't provided enough background information for anyone to be able to answer your questions.

"How do I record Change In Value in my 401(k) account?"

A typical 401(k) account in the "accumulation" phase (you're still working) would change its value based on:

1. Contributions to the 401(k), which could be contributions based on withholding from your paycheck (amounts you're setting aside), or employer "match" contributions. Depending on the details of how this 401(k) is established employer matches could be taxable or non-taxable.
2. If the 401(k) funds are invested in traded securities, changes in the per-share price of those securities would affect the value of the 401(k) account.
3. If the funds in the 401(k) account produce income in the form of interest or dividends, that income would affect the value of the 401(k) account.
4. If securities in the 401(k) account are sold at a gain or loss and the transaction incurs a fee (e.g., broker fees) that would change the value of the 401(k). (It would also change the nature of the gain or loss from "unrealized" to "realized.")
5. If the financial institution holding the 401(k) account charges some sort of periodic fee, that would change the value of the 401(k) account.

So which one of these possible "changes in value" are you referring to here? Maybe all of them?

"What Action can I use for both gains and losses?"

Within Quicken Investment Accounts the word "Action" is a selection you make when entering a transaction. Your selection shows up in the "Action" column of the Account and the selected Action allows the program to prompt you for the information needed, and then do the proper accounting when all the information is entered and you click on Enter/Done. So the needed Action is transaction-specific, depending on what "gains and losses" you're referring to. However, if you're referring to changes in security prices, that doesn't need an "Action." You simply change the per share Quote/Price for each security in the Account as of the date of that quarterly statement.

"It doesn't come from my employer. It doesn't come from me. It doesn't come from any other account. It just shows up in my quartetly statement (like magic!)."

That statement suggests to me that you're referring to a mixture of "contributions" and, possibly, changes in security prices. if that's correct then there's no "magic" involved here, those contributions are coming out of your pocket and/or from your employer, You need to understand that yourself, and then tell us where that money is coming from. As a stop-gap until you figure it out, you could simply use the "Deposit" Action and in the "Category" box enter the name of your 401(k) Account in Quicken surrounded by square brackets: [Name of 401(k) Account]. This WILL "magically" get that cash into the Account.