Internal Vacuum Pressure

Even the slightest vacuum will draw the sides in, damaging the tank. If you are going to draw vacuum on a tank, use a spherical tank.

If the walls are strong enough, a cylindrical tank can be just fine. Admittedly, a spherical tank is stronger (per unit thickness).

A good visual on why this is not a good idea:

http://www.youtube.com/watch?v=Zz95_VvTxZM

Cylindrical tanks are often used for vacuum, but your tank is large relative to its wall thickness, so significant vacuum will collapse it. You should not attempt this without having the tank professionally evaluated, because any slight flaw in the cylindrical shape will tend to encourage buckling.

10 mm water head

and sneezing near the tank is not permitted

You must have stiffeners! Without them, very little vaccum (as mentioned before, not more than few cm of water column...) should be permitted. The vessel will buckle in the middle for the slightest non-uniformity of the construction or the steel. the thickness of 4 mm is too small for the length of the tank. There are Norms for this type of construction and a professional specialised manufacturer should be consulted...

Vacuum in vessels is handled as external pressure, so you could calculate the maximum negative pressure inside your vessel or external pressure, using the ASME BPVC Section VIII, paragraph UG-28, you need to know the shell material in order to calculate the external pressure.