Four seconds off generally means there are some fundamentals that are being overlooked. A lot of good advice has already been given, so I will offer some alternatives.
Look outside GT Sport for racing techniques. It's really hard to get better if you don't know what you should be doing.
Great info here. There are literally hundreds of books/websites about driving/racing skills.
If you haven't already, do some "real" racing. I know this is not possible for everyone, but if you have the opportunity, I highly recommend it. I learned a lot from indoor rental karts. It helps to feel the forces involved to better understand how driving fast works.
Relax. For myself (and probably a lot of others,) it's easy to get tense while playing and trying to go fast. Being tense causes mistakes. Watch videos of the Aliens, they make it look effortless.
Sounds weird, but where you look can help you relax while driving. Don't focus too long on a single point. If you are only starting at the braking point, you have less time to focus on the turn in, then less time on the apex... This causes your brain to be stressed since you have so little time between each point. Notice where the braking point is, but then look ahead to the apex. During braking, look past the apex to the exit. Always have your focus well ahead of where you are. It takes a while to trust yourself, but this is a fundamental of driving.