CMOS OR gate using 4 Transistors
No. OR requires 6 transistors. NOR can be implemented with 4.

You can't put NMOS on top in a simple digital circuit because there is no voltage available to turn it on.

You can put NMOS on top on a linear analog circuit, but you will not be able to drive to the upper rail, unless there is some higher voltage available to drive the gate. If you need to drive all the way to the upper rail, you need PMOS on top.