Why is the Crucifixion said to have been between 30-33 AD, if Jesus was born between 4-6 B.C. and lived to be 33 years old? Here are the facts as far I know: 


Jesus must have been born between 4 and 6 B.C. to precede Herod. 
Jesus's ministry begins when He was about 30 years of age
He engages in a 3-3.5 year ministry 
He is crucified under Pontius Pilate 
He therefore lives to be around 33 and a half years old 


Therefore, shouldn't the year of His death be around 28-29 A.D.? 

Why does Wikipedia state: 


  The crucifixion of Jesus occurred in 1st century Judea, most probably between the years 30 and 33 AD. 


Is there that much ambiguity as to the "about"? 

I realize there is further complication because Luke states that the "15th year of Tiberius's reign" is when John the Baptist begins to preach. Moreover, there is no scholarly consensus as to whether Luke marks the beginning of Tiberius's reign to when Tiberius was co-regent with Augustus (11 A.D.) or when he takes full possession of power (August of 14 A.D.)

What is the scholarly consensus as to how to how these dates are synthesized?
Reading the  Wikipedia article cited in the question a little further we come to the Chronology section. In this we read that  most scholars consider the most likely date for the Crucifixion  to be April in the year 30 AD.

If Jesus was born in the Autumn of 5 BC, for example, He would have turned 4 in the year 1 BC, and turned 5 in the year 1 AD. (There was no Year Zero.) Having turned 5 in the year 1 AD He would turn 25 in the year 21 AD and would have celebrated His thirtieth birthday (if indeed He celebrated birthdays) in Autumn of the year 26. 

If He began His ministry in Autumn 26, very close to His 30th birthday, then three years later He would have turned 33 in Autumn 29, and still been 33 at the time of the Crucifixion, and Resurrection, in the Year 30 AD.  

On this basis His ministry would have begun in the 15th year since Tiberius became co-emperor.

We do not know for sure any of the dates, but there is no inconsistency in the assumptions made.