Using P-Channel mosfet with arduino
For your current circuit:

Why is R4 really necessary?

You need a way to make sure that the source and the gate of the PMOS are the same potential to keep it OFF. That way, when Q1 is not ON, R4 will guarantee that the PMOS remains OFF. Remember that a MOSFET is turned on when there is enough potential difference between gate and source, \$V_{GS}\$. In a PMOS, you need to pull the gate voltage, sufficiently, below of that of the source in order to turn it on. In your circuit, when the PMOS is ON―which happens when Q1 is ON― \$V_{GS}\approx -V_{in}\$ (a little less because of the drop across Q1).
Another good point brought up in the comments, without R4, when Q1 is ON, it would short \$V_{IN}\$ to ground. That is, of course, if you were to replace R4 with just a short circuit. If you took it out, leave that open, you wouldn't short VIN to ground but the previous paragraph explains what could happen.

What's the use for Q1 , I have seen diffent designs using only Q2, if I was driving a 5v device (the same as V as the arduino could Q1 being removed?

Yes, you could live without Q1 under this case you mention. If VIN, however, were some voltage like 12V, then you still need Q1 to protect the arduino's GPIO from the higher voltage.