Joy Robson died at the Snowman Rally in Inverness in 2013.
The following year, Iain Provan, Elizabeth Allan and Len Stern lost their lives at the Jim Clark Rally near Coldstream.
The inquiry at Edinburgh Sheriff Court is expected to last several weeks with dozens of people due to give evidence.
Plans to hold the hearing were confirmed late last year.
The decision followed a change in legislation to allow for a single fatal accident inquiry to be held in relation to deaths in different parts of Scotland.
The Crown Office said at the time that the two events held "similar questions in relation to spectator safety at rallies".
It added that the Lord Advocate considered that it was in the public interest that the issues be explored together.
The inquiry will look at the full circumstances surrounding the deaths and "help to avoid such incidents happening in the future".
Andrew Henderson, from Thompsons Solicitors, who is representing Mrs Robson's family said: "Joy's family greatly welcome the beginning of the fatal accident inquiry which is the first to be held since new legislation was brought in to improve the FAI process.
"They want to finally know the full reasons behind Joy losing her life and to make sure that spectator safety at Scottish rallies can be improved to a level where no other family can suffer a bereavement in such an awful and needless way."
Last year The Crown Office confirmed that no criminal proceedings would be brought in relation to either event, although that could be reconsidered if additional evidence comes to light.
The Jim Clark event has been suspended until the completion of the FAI.
The event had been held annually since 1970 to celebrate world champion Clark.
A joint fatal accident inquiry into four deaths at rallies in the Highlands and Scottish Borders is due to begin in Edinburgh this week.