Try for baby number 4 or not?

I have 3 children already a 4yr old son a 20m old son and a 3yr old daughter.   I would love to have one more, hopefully a girl so my daughter could have a sister. I feel that one more will complete our family. Should I wait a couple more years Or try now so they can all be close in age. I'm 26 
Just in case anyone wants to know

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