Journal #4 is a supposed 4th journal in the series of journals written by Ford Pines that made a cameo in Gravity Falls: Lost Legends, and is currently owned by Shmebulock.
History[]
When the reader (portrayed as a traveler) is welcomed by Shmebulock into his home inside a tree, a bookshelf can be seen in the background containing Journal #4. It is never mentioned by Shmebulock himself or ever referred to in the show or any other media.
Continuity and Speculation[]
While conducting his research in Gravity Falls, Oregon, Ford Pines created a series of journals to catalog his research. In the show, Journals #1 and #2 are shown to be complete. However as stated multiple times by Dipper and Mabel throughout the series, Journal #3 was never completed by Ford since he hid it in a rush to hide the instructions for the Interdimensional Portal. This was what allowed Dipper to log his later findings and adventures in the rest of Journal #3. This means that Journal 3 was the last one written by Ford.
In Gravity Falls: Lost Legends, Shmebulock states that he is a fan of the Pines' adventures, and that he has been watching them throughout the summer. It therefore is possible that Shmebulock created this journal as a replica of them. However, other theories suggest Ford might have wrote about what he saw on the other side of the Interdimensional Portal, due to Shmebulock mentioning him digging trough the Pines family's trash. This would then imply that Journal 4 was written by Ford during his travels or afterwards began to be written by him, but given up on. It's also possible it was a leftover unused journal from his initial years in the town and was thrown out while cleaning out the Mystery Shack.
Due to Lost Legend's canon status being regarded as "canonish," Journal 4's overall canon legitimacy is debated and reliant on whether the reader sees the book as canon or not.
Dipper's journal[]
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Dipper's journal as seen in the canon version of Journal 3.
Dipper's journal as seen in Lost Legends.
In Journal 3, on Dipper's last page, he states that he is starting his own journal. However, Dipper's journal has a pine-tree symbol (as shown in both Journal 3 and Lost Legends) so although he does own a journal, it is not likely/improbable that he owns this particular journal.
Relation to the show[]
Alex Hirsch states in a cipher hidden in the graphic novel that "it's only canon-ish". The idea of Shmebulock owning an unmentioned continuation (or replica) of Ford's Journals could be considered canon to the Gravity Falls universe since it wouldn't be too far fetched. It also could just be a little reference from Alex Hirsch to the fans to get them speculating about the extent of Ford's studies.
However, this is up to the viewers and the fans to decipher if they consider it canon in their own headcanons.