User alias beyond 4

The admin API documentation indicates that you can set aliases on the user account:

It references alias1…4, however the number of aliases was recently increased to 8. Am I correct in assuming that we can now send alias5…8 and set the remaining four aliases as well?

Yes, they are just in a different parameter now.

The individual parameters alias1 to alias4 are still included for backward-compatibility.

The new parameter that lets you set all aliases 1 through 8 is aliases.

From The Create Users section of the Admin API doc:

Username aliases for the user. Up to eight aliases may be specified with this parameter as a set of URL-encoded key-value pairs e.g. alias1=joe.smith&alias2=jsmith@example.com . Ignores alias position values not specified.

So in your POST you could include something like this to add aliases beyond the four you have set already…
aliases=alias5=something&alias6=somethingelse