What is the difference between 3rd generation and 4th generation HIV test?

The 3rd generation tests look for antibodies (the body’s immune response), 4th generation tests look for both antigens (proteins from the virus itself) as well as antibodies. This means the fourth generation test is more accurate in a shorter time as it does not only rely on the speed at which the body can generate an immune response.
As far as the numbers are concerned. Anything below 0.25 is negative. No-one will get 0. Your result of 0.06 means you do not have HIV.

But according to WHO research test reveals..

“Fourth generation assays for the detection of HIV-1 p24 antigen and HIV-1/2 antibodies have the potential to identify infected individuals earlier in the disease course, including individuals in the seroconversion phase (window period characterised by presence of HIV-1 p24 antigen and incomplete HIV-1/2 antibodies) and acute infection (characterised by presence of HIV nucleic acid and/or HIV-1 p24 antigen but not HIV-1/2 antibodies). These assays are generally of superior seroconversion sensitivity to assays of earlier generations. Therefore, they should be considered as the first-line (screening) assay where feasible.

However, recent data show that the HIV-1 antigen detection component of some fourth generation assays may be lacking in sensitivity.”