I plan to be in Spain in early September. I have about 4/5 days (for the road trip) and would like to start my journey in Seville and end it in Barcelona ( I will be in Barcelona for a couple of more days). I would like to visit Cardoba and Granada on the road trip, any place else I should go? Some suggestions about the route and any other ideas will be be helpful.

With that amount of time you won't have much time to make any detours. We have driven that route before and love both Seville and Granada. If you simple plot the route Seville-Granada-Barcelona, that is going to be about 12 hours with no stops. The plotted route will put your through Valencia which would be a great stop. Otherwise the other detours I would consider would either a beach/seaside one at Malaga or go more inland from to Cordoba and then turn south to Granada.

We have traveled extensively in that area by car and it is lovely. Hopefully by September the tourist traffic will have died down by the coast-we have always gone off season in the spring but I do know summer travel at the coast can be very heavy-just like most of Europe at the seaside during holiday time.

While it depends on what you want to see and experience, I thought Cordoba and Granada worth more than a passing glance. I had two full days in Cordoba, and wish I'd had more. I had 3 full days in Granada, which I found barely sufficient.

If you haven't already seen it, I would strongly encourage you to consult the excellent Michelin Green Guides for Spain.

Some places I recommend which are slightly off the beaten path but definitely worth stopping for a look, or even staying the night.
Cuenca is an amazing old town on the cliffs edge which is halfway between Seville and BCN. The Costa Dorada, which is much closer to BCN, has plenty of nice beach side towns. Tarragona and it's ancient old roman ruins. Peniscola too.

I would do either Córdoba or Granada, for at least 2 nights. And a stopover (or another 2 nights) in lovely Valencia.
The tollroads (E-5 / E-15) in Spain are easy to drive and there's usually not much traffic. Avoid the national roads (N-roads) for this 1000 km journey.