Nope. The breather tube ends in the air box, on the atmospheric pressure side of the carbs. Don't take my word for it, measure it yourself. 
It is not atmospheric pressure until you get to the outside of the air box inlet port.
If it was atmospheric pressure in the air box, no air would move into it. So, by definition what is inside the air box MUST be lower pressure (partial vacuum) than the outside atmospheric pressure.
In fact, all along the path from air inlet to intake valve is a gradient pressure drop. It is caused by the falling piston, and the negative pressure created by that expanding void is filled from the air inlet at the air box entrance. All along that pathway a negative pressure exists, as that is the only reason for air movement in the duct work. If you want air to move, there must be a high pressure moving toward low pressure, otherwise there is no movement. The breather system in the SOHC4 is sources partial vacuum at a point in the air path after the air filter. The filter membrane creates another pressure drop across it. The inlet air opening provides another point at which a pressure drop occurs. Both of these points, as well as the duct distance and wall boundary layer, creates an additive pressure differential that absolutely must be less pressure than atmospheric. It is certainly measurable with proper equipment.
Measure how much vacuum is developed in the breather hose. This is NOT a PVC system, as used on cars, that tap into the high-vacuum intake manifold.
PVC stands for Positive Crankcase Ventilation. Which, as you pointed out, is NOT what the SOHC4 has. Auto PVC systems have both and inlet and and outlet the crankcase so air can flow through it. The SOHC4 has a single port. (It can employ this because the internal crankcase volume of the engine doesn't have a net change due to two pistons moving while the other two move down.) So, if you apply even a small negative pressure it will stay inside until equalized from another source, most probably blowby, which would then be fed to the engine combustion chamber through the carbs.
There are many flow meters that use the pressure drop across a calibrated distance in order to determine flow rate. No pressure drop equals no flow. This is a basic principle of fluid and flow dynamics being exploited to gain measurable information.
I admit I never measured the breather tube pressure, or the crankcase pressure, for that matter. Some things you know exist, simply because they must obey the laws of physics. I don't have to measure the weight of all things to know that if they have mass, they will be drawn to the earth.
The negative pressure can also help keep engine leaky seals from oozing oil outside, btw.
Cheers,
