Do you mean using 4lb test for the whole length of the leader? If so, try it.
You can look at the leader in the same manner as the fly line itself. It's heavy
because the line's mass is needed to make a cast. If mono had enough mass,
we could skip fly line altogether, and just cast a long leader.
The fly cast system is an energy transfer system: the rod responds to your
arm movements, transferring energy to the line, which in turn transfers energy to the leader. If you had a 9' length of 4lb test at the end of a 5wt
line, the sudden transfer of energy from a heavy line to a very light line
probably wouldn't look pretty. I'd imagine the "leader" would fail to keep up
with the line, and would quickly collapse and wrap around the line.
You can get away with using a non-tapered mono leader in larger diameters,
as with bass bugs. I tie my own leaders, and use a 4 section leader for
0X size leaders. There's still a taper, but not much.
The rod itself
is also tapered, for the very reason the line/leader are....