i'm looking at 3 different 911 4s 07-08 or 09 manual or pdk cabriolet or coupe

A Porsche has 3 pedals. Anything less is not a sports car. Pick the soft-top, you'll be glad you did.

low miles, maintenance records, original owner over 40- buy that one

Make sure it has the intermediate shaft bearing has been replaced with the new ceramic bearing. This is an expensive change, and not all Porsches will experience problems, but it is a safe bet to do so. The all wheel drive is expensive and complex to maintain, so unless you plan to drive in the snow, stick with the RWD models.

The 2009+ cars are a different generation of the 997 model, with a number of upgrades to the electronics, LED lights, suspension and PDK gearbox instead of Tiptronic. The answer to your question is only provided by yourself. What kind of driving will you do with the car? If twisty mountain roads, buy a 997 Carrera S coupe with manual box. If commuting on a busy freeway, get a PDK RWD coupe or cabrio Carrera. If you go down to the beach, get any version in a cabrio body. If you will do any track days, look at earlier model GT3 cars.

If you can afford the 2009 get that one. It has a newer design engine with the (IMS) intermediate shaft issue re-engineered. Agreed with what Smitty22 saiys above...only get the 4S if you drive in rain or snow. More expensive to maintain and hence the cheaper price (although when sold they were more money). Convertible is nice if you live in an area where you can take advantage of the top drop. Coupe is a bit more stable and has slightly better design lines. PDK is fabulous to just push a button and get lightning fast perfect shifts every time. Shifting manually is more fun for the "serious" driver. You need to address all these issues before you buy.

Fantastic answers... Do your research. And! Don't be afraid of a car with higher miles...IF... you have parts replacement records throughout the mileage cycle. For example.... 80K miles, clutch not replaced. This means the car has had a good life. And, a clutch replacement will be required with 5 to 10K miles. A 40K mile clutch replacement is suspicious. Find out why their was a clutch failure!