Why M3 not M4?
One of the most common dilemmas potential M3/M4 owners face is which vehicle to purchase. Each car has its own merits and faults. I will go over some of these in a moment.
Styling of a vehicle is paramount to a buyer. If the car doesn't look nice people simply don't want to buy it. If you're going to be looking at a car that you are going to be driving, you want to think "wow, what am nice car I get to drive" and not something like "mmm...I wish I had a better looking car". Well in the case of the M3 vs M4 in terms of looks, I don't think you can go wrong either way. They look very similar so it really is up to personal preference. The M3 has 4 doors and a less flared back end. The M4 has a slightly steeper sloped roof line along with a spoiler. The spoiler is included since the M3 is more aerodynamic than the M4. The rear wheels protrude further from the body compared to the M3. In terms of looks, there's not much to say. Unless you put them side by side, the only obvious difference is the amount of doors and the size of the doors.
If you have a family or many people who get into your vehicle often, the M3 is a more practical vehicle in that sense. The backseat is by no means large or comfortable for long periods of time but it is much easier to load passengers on and off. It has a middle seat in the back row whereas the M4 has a small storage tray in lieu of an additional seat. The roof is also more forgiving to taller passengers as it slopes down less where the back row passenger sit. The M4 only has 2 doors which makes getting passengers in and out more of a hassle. The front seats can be moved forward and backwards via switch on the back of the seat. It moves the seat faster than the buttons on the side of the seats but is not fast by any means. The back row has a smaller head space so people who fit into the M3 backseat might not fit into the M4 back seat. If you are the only one who normally gets into the vehicle then either the M3 or M4 would be fine.
If a convertible is more of your thing, then the only choice would be the M4. It allows you to fold the top up or down at up to around 25 mi/hr. If you don't mind giving up the convertible feature and a moon roof will suffice then the standard M3/M4 will work. There is a price premium for the M4 convertible but there are a few features included that neither the standard M3 and M4 don't have. The convertible will have thigh support. It is a portion of the seat that is able to extend the length of the seat so that your knees and the seat can line up allowing for a more comfort ride. Neck warmers are also available if you desire.
Another thing to consider is the depreciation of the two vehicles. The base M3 costs less than the base M4. So straight off the back, you will be spending less money to get the same specs for each car. Let's assume that there is an M3 and M4 that are exactly the same except for the model. The M3 will hold its value longer than the M4 numerous reasons. The M3 is a harder car to get in the states than the M4. The M4 seems more abundant so the M3 is more rare. In general, the M3 will be worth more on the used market than an M4. This is straight up fact whether it should be or not. It is quite a shame in the sense that you are paying more for the M4 and it will be worth less than a similar M3 that costs less to purchase.
The BMW M3 is also a cheaper vehicle to insure in most cases. The vehicle costs less than the M4 so that makes insurance rates go down by some amount. Also insurance companies look at the person who is driving, the type of vehicle he/she is driving among other things. The insurance company is thinking that a practical person is more likely to buy an M3 for the number of doors. The M4 is a coupe so people who buy it generally want to focus more on being sporty. Whether these conceptions are true or not the insurance company sure thinks so and the rates will be higher on the M4. Over the longer term you might be saving more with the M3.
This pretty much sums up the major differences in the two vehicles. If you are driving alone in most of the time and want to have a meaner looking car that stands out more often go for the M4. If you want a more practical car that can double as a family car or a monster on the roads then go for the M3.