Nissan Pulsar Wont Shift to 4th

I recently bought a 87 nissan pulsar nx, 153,000 k. The car runs mint and shifts fine until the end of 3rd gear. It feels like it wants to shift into 4th but wont. I had to drive 200 km on the freeway lastnight and was only going 100km/h at 4000 rpm for about half an hour. What could be the problem.(Im guessing the Torque Converter or coolant sensor) but truly have no idea.