Is it possible that in a house if parents are having 4 children already.... and the elder one is about 20 years almost ready for marriage and at that point parents decide to have another child?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

There is no impediment Islamically if the parents want to have children after the age of the rest of the children has advanced.

Family planning and its arrangement are up to the parents according to what yields the best benefit.

Allah Knows best.