The phenomenon of the four DeLoreans is that as of the end of Part II, there are four different versions of the DeLorean time machine present in Hill Valley for several hours on November 12, 1955.

This occurs from Timeline 7 onwards, and is not shown in the trilogy. After Emmett Brown is struck by lightning in Timeline 6 and sent to 1885, a new timeline is created where a DeLorean had been stored in the Delgado Mine for 70 years.

In each case, the arrival of the DeLorean rewrites the events of Saturday, November 12, 1955.

Zero DeLoreans

In Timeline 1, there are no DeLoreans in 1955. This is the history of the original 1985 timeline, and is not depicted on film.

One DeLorean

In Timeline 2 and Timeline 3, there is one DeLorean in 1955. This is depicted in Part I.

Marty A has been present for a week and is taking Lorraine to the dance.

35-year-old Doc is working on DeLorean A.

George is planning his fake confrontation with Marty.

There is now a repair bill for $302.57 for Biff.

Two DeLoreans

1955

In Timeline 4 and Timeline 5, there are two DeLoreans in 1955. This is the history of the Alt-1985 timeline, and is not depicted on film.

Marty A and 35-year-old Doc are present with DeLorean A.

78-year-old Biff is present with flying DeLorean B.

78-year-old Biff gives the Grays Sports Almanac to 18-year-old Biff.

1985

In Timeline 2, there are two DeLoreans in 1985, for 11 minutes on Saturday, October 26th, as depicted at the end of Part I.

At 1:24 a.m., Marty B arrives from 1955 in Courthouse Square, in DeLorean B.

At 1:35 a.m., just after the Libyans shoot Doc dead at the parking lot of Lone Pine Mall (formerly Twin Pines Mall in Timeline 1), Marty A accelerates DeLorean A to evade the Libyans, and as he does, he departs for 1955.

1885

In Timeline 8, there are two DeLoreans in 1885 for five days, as depicted in Part III.

On Wednesday, September 2nd, Marty arrives from 1955 (Timeline 7) in DeLorean D. DeLorean C has been buried in the Delgado Mine for (at most) eight months, since Doc's arrival there from 1955 (Timeline 6, upon getting struck by lightning).

On Monday, September 7th, Marty departs for 1985 in DeLorean D. DeLorean C remains undisturbed and preserved in the mine until it is uncovered seventy years in the future.

Three DeLoreans

In Timeline 6, there are three DeLoreans in 1955, as depicted in Part II.

Marty A and 35-year-old Doc are present with DeLorean A.

78-year-old Biff is present with flying DeLorean B.

78-year-old Biff gives the Grays Sports Almanac to 18-year-old Biff.

to 18-year-old Biff. Marty B and 65-year-old Doc are present with flying DeLorean C and a hoverboard.

Four DeLoreans

In Timeline 7 and Timeline 8, there are four DeLoreans in 1955, this is the history of both Alt-1885 timelines, and neither are depicted on film.

In this timeline, there are temporarily two Biffs, two Martys, and three Docs (one long dead).

Marty A and 35-year-old Doc are present with DeLorean A.

78-year-old-Biff is present with flying DeLorean B.

78-year-old Biff gives the Grays Sports Almanac to 18-year-old Biff.

to 18-year-old Biff. Marty B and 65-year-old Doc are present with flying DeLorean C and a hoverboard.

DeLorean D is buried in the Delgado Mine, and has been for seventy years.

Doc's letter has been in the office of Western Union for seventy years.

Timeline 7 only: 65 year old Doc is buried in the Boot Hill Cemetery outside the Delgado Mine, and has been for seventy years.

In the period of Timeline 7 depicted on film at the end of Part II and start of Part III, after Doc is struck by lightning, there is only one DeLorean left:

DeLorean A has departed with Marty A to 1985. 35-year-old Doc stays.

DeLorean B has departed with 78-year-old Biff to 2015. 18-year-old Biff stays.

DeLorean C has departed with 65-year-old Doc to 1885. Marty B stays.

DeLorean D is buried in the Delgado Mine, and has been for seventy years.

65-year-old Doc is buried in the Boot Hill Cemetery outside the Delgado Mine, and has been for seventy years.

Marty B is present with the hoverboard and Doc's letter, and makes contact with 35-year-old Doc.

Overlap

In Timeline 7 and 8:

Delorean D arrives 1885-01-01 00:00 (65-year-old Doc)

Delorean A arrives 1955-11-05 06:15 (Marty A)

Delorean C arrives 1955-11-12 06:00 (Marty B and 65-year-old Doc)

Delorean B arrives 1955-11-12 13:40 (78-year-old Biff)

There are 4 DeLoreans present for 4 hours and 58 minutes.