Hi Ricardo,
Apparently the Default 8 days was coming from the calculation of the Failure Days you set in the SLA plus the combination you use in the Customer Service Schedule you set in the Business Hour in your SLA Setting.
Okay as per example here:
I didn't set any Business Hour (so according to CRM it was considered as 24x7 service, means non-stop, okay).
Then I set the failure to 10 days here.

Remember, no Business Hour nor Holiday Schedule I set.
So here is the result as per expected, resolve by 10 days from the since its creation date

Then what if I set the Business Hours right now and create my CS Schedule like here:

Actually with lunch break as well (30 minutes)
*Note, remember I didn't set any Holiday schedule, but I set Sat and Sun as the weekend day so no support there.
And here is the result:

As you can see the resolve by will follow the rule, plus the 10 days from the created on then also considering you are not working in Sat Sunday and you are not working 24 hours (1 day full), in fact you are only working 9 hours with 30 minutes break, CRM calculate it, so the final calculation will be the final resolve by.
Okay so now between the day of createdon and 17 March 2015, I wanna to create a holiday schedule in between so that it should also increase the tolerance of duration to resolve by the case.

That is Chinese New Year 1 day on 19 February 2015.
We are expecting that the Resolve By should increase 1 day as well right since in between those days, your CS employees are not working and should be expected to resolve the case as well.
So here is the result:

*Yes, as per our expectation based on the Pattern, it was increased to 18 March 2015 caused by a holiday set here.
So, Ricardo, there is no special exact setting to set 8 days or 4 days here, because it is dynamic, combination from Failure tolerance days, Business Hour, and Holiday Schedule.
If you have no setting of Business Hour nor Holiday Schedule then you can rely on the Failure setting, if not then it will follow the rule and the CRM calculation, I think it is the expected behavior right.
If you have no setting of CS Calendar schedule (Business Hours, Holiday schedule), you can set the failure setting 'failure after' just to 4 days and assuming your CS working 24 hours without weekend.
Hope this helps.
Thanks.