Which statement about keeping a safe distance is correct?

1. ①The faster you go, the shorter you should keep the gap so other cars cannot cut in.
2. ②When it rains the road surface is washed clean, so you may keep a shorter gap than on a clear day.
3. ③If the vehicle ahead is a large truck, follow closely behind it to reduce wind resistance.
4. ④Increase the gap at higher speeds or on wet roads so you can avoid a collision even if the vehicle ahead stops suddenly.