These are two concepts, The bending tensile strength 4.0MPa is the bending tensile strength, that is, the bending strength 4.0MPa, C25 and C30 refer to the compressive strength reaching 25MPa and 30MPa, The two concrete design processes are different, and the flexural strength is generally used for pavement. But you have to connect them. The concrete with bending tensile strength of 4.0MPa is not only C30, but can reach C35.
At present, the most commonly used ones are those with the nominal value of 31.5, while those with the nominal value of 37 are used, but few are used. The nominal value of 26.5 is generally used for structures such as beams and slabs with small section and dense reinforcement
