Are 4 yr olds only supposed to have 20 teeth why is this?

Every night my son brushes his teeth while I brush mine, and sometimes when he's done I'll go back and rebrush. and for the first time I counted his teeth and he only had 20 teeth. Now before you think I'm a meat head I'm an engineer not a health care or dental professional. Because It seriously don't look like hes gonna fit any more teeth inside his mouth any time soon. so when will he get all the rest of his teeth in, just askin.
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All of his permanent teeth may not be in until he's in his teens.
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