Well, one of the things you could try is rotating toys: observe which toys aren't being used on a regular basis and put them away for awhile; when you bring them out again, put others away. Do this every so often and you'll probably notice your children looking forward to the change. That's one way to control toy clutter and provide the right balance of toys.
Also, try helping and encouraging your children to put their toys away after playtime, with a special place for everything. Doing this will not only help them to keep toys orderly, but the process itself will teach them the beneficial skill of how to categorize things. When toys are put away so all the pieces that go together are stored together, I have found children are more respectful of their toys and they actually play better! When you can't find a place, or places, for everything in an organized way, that is a sign there are probably too many toys out, so put some away for another day.
Since your children are several years apart in age, finding the right balance of toys may, at times, be a challenge, because what is interesting to one child at the moment may not be of interest to the other. However, there are many toys that have long-lasting play value and will engage the interest of both of your children. By the way, since your daughter is 4, some of the toys that are appropriate for her will not be appropriate for your 1-year-old son. Therefore, your daughter should be encouraged to help keep toys with small parts out of reach of her younger brother.
Our parenting advice is given as suggestions only. We recommend you also consult your healthcare provider, and urge you to contact them immediately if your question is urgent or about a medical condition.