Order by FIELD function of MySQL
You should be able to just use `fn.FIELD()`, since FIELD is a sql function:

```python
my_query = User.select().order_by(fn.FIELD(User.id, 4, 1, 3, 2))
```
MySQL support FIELD function to sort result with specified order. But I didn't find the usage in peewee. I wonder that if peewee support that?