While it is good to use tires with the same tread and sizes, it is possible to use tires with different tread patterns, though you should be careful as the performance of the two tires may be different, causing your car to be unbalanced, endangering your passengers.
the tyres could blow and you may lose controle of your car
If you want traction you need the tread. Smooth tires are only used for racing.
So the tires will "wear" evenly and you will have longer "tread-life"
If your tires do not have enough tread left then they can skid easily because there are no edges of a tread to grip the road surface. It's the edges of the tread that actually grabs the road the best and not the flat rubber. If you have good tread then perhaps your car is over powered for the tires that you have. Check with your vehicles manufacturer as to the recommended tires. The best thing would be to talk to a tire shop. They could give you the best information.
4/32" on the front tires and 2/32" on the rear tires.
smaller cross-section of tread in contact with road
It means it is a tire for a passenger car and tread is 235mm wide.
It depends on the legislation of the country in question
It would also depend on the road conditions and tread pattern (if any) on tires.
Eventually, you will notice that your car is losing traction due to tread loss, especially on wet surfaces. You'll also notice that you may not be getting the mileage out of your car that you used to. A good, simple way to check the tread is with a quarter. On most tires, when the tread is lower than Washington's nose, it's time to take your car in and see of you need new tires.
On or before your country's legal minimum requirement for tread. If in doubt see out a qualified fitter to check the tread depth for you