You need to add to your query a 'where' clause. The SQL becomes
update mytable
set recordchanged = current_timestamp
where id = id_of_last_changed_record
It's best to pass the id of the last changed record via a parameter, making your code
with ABSQuery4 do
begin
Close;
SQL.Clear;
SQL.Text :='UPDATE MYTABLE SET RECORDCHANGED=CURRENT_TIMESTAMP WHERE ID = :P1';
sql.params[0].asinteger:= id_of_last_changed_record;
ExecSQL;
end;