If you have a 2nd baby while your 1st is still under 5 yrs old, you are selfish. You are just filling your desire to have another child and not focusing on what best for the older sibling. That child should have at least those first 5 years of life to bond with the parents alone.
The first 5 yrs os a child's life are extremely important to their emotional development and they need their parents there for them 100%. To have another baby and basically tell them, "It's time to be a big boy/girl because mommy is having another baby." while they are still going through this early stage of development is cruel.
If the older child is 4-5 yrs old, they will be out of the house, in school during the day and now you can focus all of your time on the new little one.