Lau seme-alaba ditut. Seme=son alaba=daughter. As you don't know beforehand if they are male or females we say seme-alaba. If you know for sure that they are guys, you say "lau seme ditut".

I have 4 children: Lau seme-alaba dut (edo ditut) (Zenbakiek aditz mugagabea eskatzen dute baina onartzen da mugatuan egitea.)