[Hope]

No one can determine how old was Rebecca because the Jewish Bible doesn't record it. How the Jewish scholars came up with 3 or 4 years of age? They did some maths based on the information available in the Jewish Bible. 

Prophet Isaac, pbuh, could have married her at 4 years of age but he probably waited till she was sexually mature before consummating the marriage.

Wallahu 'alam.

Even David married an underage as per terms of 21st century.Some of the prophets had concubines.