Sorry, I can't let this misconception go on. First, isn't it logical that a 2x4 be, 2"x4"? Today the measurement is "nominal", see the note at the bottom of this post.
As a kid in the late 50's and early 60's real 2" x 4" dimension 2x4's could still be found, they also had tighter grain, few or no defects like wane (bark or rounded edges just under the bark that are less than dimension) splitting and few or no knots. I helped a neighbor on many projects from rec-room, bedroom addition, sheds and boats and we saw that lumber was shrinking every couple of years, I know because we measured what was being sold as 2x4's. First it was 1 15/16" x 3 15/16" then 1 7/8" x 3 7/8", later 1 3/4" x 2 3/4", etc. and it has shrunk even further from the 1 1/2" x 3 1/2" you speak of, it is not uncommon to find 1 7/16' x 3 1/2" or even 3 7/16". And these dimensions are for wood that is still fairly wet and will warp and shrink some more as it dries further.
If you could run that new 2x4 back through the mill to see what the initial dimension was, it would not be 2" x 4", I can't give you the exact figure but you can find that the larger tangential shrinkage rate for Douglas Fir is 7.6% (pine has a lower percentage) from an approximate 30% moisture content to oven dry conditions, kiln dried wood may not be this dry and lumber mills do not ship oven dry, framing lumber is what we call "green" or air dried. Wood cells shrink as they lose water which accounts for less than 1/8" (1.619") in the 1 1/2" dimension and a hair more than 1/4" in the 3 1/2" dimension (3.766"); lumber mills almost exclusively use band saws with kerfs (the dimension of the saw cut width) of 1/8" or less.
Bottom line: this isn't your father's, or grandfather's 2x4. Please feel free to insert "mother's" where appropriate if you feel the need.
Do you want to know why? Money. You can get more lumber out of each tree if the lumber and/or the tree has smaller dimensions. And this has been done even as saw blades got thinner and computers optimized sawmill practices that improved cost/resource efficiency. The price of 2x4's has kept going up even as their cubic foot volume shrank. What we lost: thicker dimensions provide higher structural load capacities and a longer burn through time. What we gained: our natural resources go further and lumber barons make more money.
Nominal Size - As applied to timber or lumber, the size by which it is known and sold in the market often differs from the actual size.