how to now what button was pressed before!

Question

I am a beginner and would like some help.
I have 3 button and the action on the 4th depends who had been pressed before.
try to give me some suggestion
thanks in advance!!!

button4 on-click
if(button1 was pressed before)
{
   do something
}
else if(button2 pressed before)
{
 do something
}....