Hi Sergio,
The F4 BAdI will only be triggered if you push the value help (i.e. F4 button). So you cannot supply default values this way. If you want to supply default values before you do anything on the variable popup, then the customer exit variable (i_step = 1) is the only option.
Using BAdI RSR_VARIABLE_F4_RESTRICT_BADI you have (changing) parameter C_T_RANGE available. Although I did not try it out myself, it think it should be possible to overrule / overwrite the default values here.
Best regards,
Sander