Well, filtering dropdown lists is certainly on the horizon for Method. We'll be adding that functionality in the near future. However, in the meantime, you can set up a conditional warning only allows users to select accounts that begin with 9999.
You would need to add this to every Save button and Save & New button on the screen - which on the Check screen there's 4 total. I'd start with trying it out on just one though.
Your action set would look like this on the Save and Save & New buttons - noting that these 3 actions would have to be the first three actions in the action set, followed by the existing actions for the Save and/or Save & New functionality that already exists for the buttons on the screen:
Retreive Value from Table - assign the account number (pulled from the account table) to an action result for use on this screen.
Character Function - set the action result to Yes/No starts with? - typed in value of 9999 and then place the resulting value in a new action result.
Conditional Warning - set a conditional warning that is triggered when the result of the the new action result is not equal to the typed in value of Yes.
Then, when the user clicks the Save button, if the account number pulled based on the Account they selected does not start with 9999, the warning message pops up and they are forced to select an account that does begin with 9999.
To make things a bit more confusing, there's also a Save All Sections action built into the properties of the Pay to the order of dropdown list. So, you may need to add the action set to the Pay to the order of dropdown as well if you're having problems.
Hope this helps!