How did you know you were finished having baby’s?

I have three daughters (5, 4 and nearly 2) and one angel son (I had to have an arranged medical miscarriage as at my 12 week scan he was diagnosed with fetal acrania which they said he wouldn't survive for very long after birth). Anyway, how do you know you are finished having babies? I feel like I don't know whether I want another baby because I'm wondering 'what if' or because I genuinely want another baby. I know I am very lucky with the three I already have but I feel like if I was to have another baby I would like to have one soon seeing as my youngest is still small. I need some advice on how to know what to do.