For many years, I have followed the idea- if it isn't documented, it didn't happen. Did you get the delivery date in writing? If so, that is your starting point.
4 weeks isn't an incredibly long time, but I can also see it being an eternity when you need it and, in this case when you put money down on it.
Any number of factors could have come into play once the order was placed however, the salesman has the ability to call the manufacturer to find out the cause of the delay. This is just good customer service.
Difficult situation to be in...... If you are persistant, then there is always the possibility they will "remember you". I do not envy your position at all-
If you are not able to gain satisfaction locally, I would try calling the manufactuer directly to check on the status of your order.