No More Than 4 Employees Allowed On Form

On my form I need to limit the number of employees that can be entered using OnCall as the criteria or use ExpName or Employee ID

If this criteria is NOT met then disable On Call. In other words if they try and enter a 5th employee then the field On Call/checkbox is disabled

name of field is OnCall and actual control is named On Call

I need this to fire on the OnCurrent Event for the form which is named frm_Employees_on_Call_What_Order

In other words, no more than 4 employees are allowed to be on call at the same time. The fields for the query the form is based on are:

All are form the Employees table

Employee ID Employee autonumber primary key
ExpName name of employee, includes first and last name
On Call Yes/No checkbox used for if a employee is on call or not

query used is named qry_On_Call