April is already spring, and the temperature is around 10-20, so it is enough to bring a thick coat. But there is a lot of rain, so you need to bring waterproof tools. In addition, the continental climate temperature difference is large, so a thin vest can be added, and the maritime climate temperature difference is small. You can wear sneakers.
If you want to go to Northern Europe or Central Europe, such as Britain, Sweden and Iceland, you'd better wear woolen coats, scarves and boots. If you are going to France or Italy in southern Europe, just wear some sweaters and coats.