So these are 4 year old saplings. They didn’t loose leaves when I began growing them, I thinks it's due to being inside all the time. I just want to make sure, if they're losing leaves due to winter or do they have some sort of disease?
The answer to this question will depend on the time of year (in this case november) and your region.
For many regions in North America and northern Europe, november is a normal time for trees to start changing into autumn colors. Yellow leafs are a very normal autumn color for an apple, but I've also seen some apple trees show some red leafs.
The difficulty by going with the picture alone is that yellow leafs can also be caused by overwatering or underwatering (although less likely). If the timing is off for your tree to start showing autumn colors, you likely are overwatering it.
Lastly, I would say it's unlikely that this is caused by any kind of disease. Leaf decoloring can be a sign for either a pest or fungi, but in the case of a pest you would generally see other kinds of damage to the leafs, and in case of fungi you would generally see other kinds of decoloring or patterns.