well, try this, this is your code in the correct format:
on 4:TEXT:#topic*:*:{
if ($nick isuser 4) { topic $chan $2 }
else { notice $nick test }
}
notice i changed the {} after the IF statement to (), this is the correct format. Also, there is no ISUSER in mirc, if you mean a regular user (no op/voice), then it would be ISREG. I am assuming you dont want ops to to this, otherwise it would be ISOP for ops (@), or ISVO for voices(+). If this isnt working or isnt what ya want please reply and state what the command is supposed to do