To add a custom wage code:

1) Start Payroll 4
2) Click on "Company info & Options"
3) Click at "Add" on on "Wage code"

The Wizard starts.

4) Click on "Next"
5) Enter a code name and a description, for expample:

Code name: MYCO
Description: My custom code

6)  Click on "Next"
7) Select a code type

You can choose form:

"Normal salary component" - Regular wages
"Wage in kind (Loon in natura)" - Free housing, free meals, etc.
"Extraordinary wage" - Once a year extra like a bonus
"Untaxed expenses" - Very rare
"Overtime" - Regular wage, but not insured for the SVB / SZV
"Payout method" - New bank
"Non-disclosed wage components" - Loans, third party deductions, etc.

8) Click on "Next"
9) Select how the amount is calculated

For example: "To be determined amount"

10) Click on "Next"
11) Select "Debit" or "Credit"

12) Click on  "Next"
13) Click on "Finish"

See also:
https://www.youtube.com/watch?v=A2z4E7VHFEo