At the age of four weeks old, kittens are usually ready to eat solid, chunky kibbles - by that age their baby teeth should mostly be already there, enabling them to do so (source: pets.thenest.com). In principle, the introduction of solid food should be gradual, so that they could get used to the different texture and hardness at their own rate. However, considering your observation that they seem to be capable of chewing it right away - I wouldn't be that concerned and I'd say it's all right to let them access this type of food.
On the other hand, please don't replace their diet completely and suddenly to solid kibbles. What I mean with the first paragraph is that it's all right if they go for them if they fancy, but they absolutely need to continue being fed a soft, wet, easily chewable kitten food as a base of their diet.
Also, please consider that the optimal nutritional profile of adults cats and kittens are different - for example, kitten food tends to be higher in fat, protein and general caloric content, but lower in carbohydrates (source: theidlecat.com). I'm not really sure whether the "mother and baby cat" food formula you're using accounts for that, so please keep their different nutritional needs in mind.