Should I Buy a used BMW 4 series 16 plate now? Or should I wait for Until after the new mode is released?

None! BMWs are money pits and cost money to run and maintain as they age. Invest your money in a Camry, may not be as luxurious but you definitely keep more money your pocket vs maintaining a BMW! My cousin/mechanic deals with German cars all the time and he tells me about their maintenance costs and garbage engineering.

Buy a Camry and you won't regret it!

I doubt if a 4 year old car would be massively affected by a new model.

I’d go for a 435d for a bit more money. The performance is incredible. Better still if it has X drive.

Dealers will be keen to sell the new models as soon as they come out, so, to encourage S4 owners to buy the new model, they will give generous part exchange values to S4 models. So, you could find, that for a period, the older models will be worth more, not less.

have you seen the new 4 series?   its as ugly as hell with that extra large grill ..i would not buy it just for its looks ..nor will many ..you are old enough to make your own decisions 

And if you wait until next year, there will be even more depreciation.

If the new model is introduced in Oct. Prices for the present model won't fall immediately. It will take at least a couple of months. If you can wait then you may save. But that means you will be waiting at least 5 months. (I tried to like that car but I just think its a bit homely)