It looks like two small code bugs:

1:  there is an extra space between "GetFieldValue" and the parenthesis:

Change: GetFieldValue ("COUNTER");  to  GetFieldValue("Counter");

2: the predefined Alert function in most browsers is lowercase:

Change: Alert  to  alert

Note that depending on how field COUNTER was deployed, the default value might be 0 (instead of empty). This might be exactly what you want but, just note... Any value less than 4, including 0 and negative numbers, will enable REASON, and any amount 4+ will disable REASON. You might need extra bounds checking such as (amount > 0 && amount < 4).

Jraile is correct on GetFieldValue.  Using upper case for the field name tells SBM to look for the database field name which is "REASON" in your case.  the Camel case "Counter" indicates the display name of the field, so if your displayname was all lower case you would use "counter".

Thank you Jralle, I have corrected error you mentioned, and it works.

Thank you David!