With construction on Terminal 5 ongoing it is not possible to walk along the terminal fronts to reach 4. The pathway that connects 6 and 4 crosses through the parking areas and takes about 5 min.

I suggest jumping the AirTrain for a nice overview of the airport, especially a nice view of T5 in its new state, if this interests you. Also a good view of this from the T6 Airtrain elevators on the 3rd floor. For an "interior view"(ops side) of T5 by jumping the annex terminal shuttle(Gates 19 and higher) which has been set up right in front new terminal 5.

You can also walk, if interested along the perimeter of T5 although the only views of the new terminal will come from the area closest to T6.