Most ducks lay poorly or not at all in the winter. I think its a combination of cold weather and short hours of light, possibly feed as well. In the summer they have at least 14 hours of light, warmth and things like insects for greater protein and greens for vitamins.
Some breeds, which are bred to be good layers, will lay throughout the winter. Some individuals will also.
Most poultry, as they become older (past two or so), become more seasonal in their laying. I have some older ducks that lay only in the spring and summer that used to lay all year. You said yours were four years old, they will probably not lay in the winter. I would not feed them layer feed when they aren't laying. I do keep oyster shell available for those that do lay in the winter. Also ducks take a break when they are molting, it would be hard on their bodies to grow feathers (they use a lot of protein) and to lay at the same time.
Breeds that are bred for meat have not been bred to lay all year, because most people want their ducklings in the spring and summer for ease of raising them. It is hard to breed for more than one trait and really make progress. Therefor most Pekins don't lay that well, but most Khaki Campbell do. Because brooding keeps a duck from laying, most breeds bred for laying have had broodiness bred out of them. Many of the meat breeds do brood, it's a useful trait in a meat duck.
None of these are specific, there are always some meat ducks that lay great and some layers that take time out to brood (but you don't get as many eggs a year from those!). There are some breeds that try to do both, they won't be quite as good as the specialists, but can be useful all purpose birds for many situations.
I hope that helps!