My car scares me while changing gear from 4th to 5th gear because it goes to 3rd gear. As a result on highways i get scared to change to 5th gear.
Can i drive my car in 4th gear at speed range between 100km to 115km ?
Yes you could, but it will cruise happier in 5th...
Nothing puts a strain on the engine like forcing it to pull the car in the wrong gear. Too high a gear strains the bottom end of the engine around the crank and con rods. Too low a gear will mean you are revving more than you need to so the top end of the engine around the valves, cams and lifters are working too hard. You need to troubleshoot why your car goes back to 3rd gear when you engage 5th gear.
Most 5th gears are over-drive, for cruising at lower RPM to get better gas milage. Older, pre-80's, cars did not even have overdrive and the result was simply more fuel burned cruising the highway at 2500 RPM. The engine will not overheat at a modest RPM under the little load needed to simply maintain legal driving speeds. Many pickup truck automatics have a no-overdrive setting "tow" mode allowing intentional driving without overdrive. Botton line: If the car is not getting hot, you're ok. However, a faulty gear selector is hazardous; you could find your transmission locked up during a shift.