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Update:

Apparently this is not yet the solution to the puzzle, nor is the number 5 or 6 (the rotation used for the last (truth-)grid/the number found written in the last grid).
I'm not sure whether the hint 3+4+5=6 still needs to be used at this point..
Another way to go could be to start looking for the truth, since that's what the previous part actually suggests. Obviously the word truth was found in the fourth and last grid, which also has the word "six" in it, but we already discarded that possibility. Maybe we should go by the saying "the truth lies somewhere in the middle", leading us to the centre part of the grids/crossword, but that's just guessing and it has no logical basis.
The latest hint seems to suggest looking at the crossings of the crossword puzzle, which would be the letters H R T D O E A D I Y, but that doesn't really help either. Maybe we should focus just on the only real crossroads, the one where 3d and 6a cross around the letter O, but I have no idea what to do with that..

N.B., something that does add up nicely:

New update:

The latest hint suggests to use alphametics to solve the puzzle. Since we're given the clue 3+4+5=6, we probably have to sum, the words found in the corresponding grids.

This means:

WHERE+IS+THE=TRUTH, which has the following solution: 69404 + 81 + 794 = 70279. So the value we place on truth is 70279.

Previous update: (going in the wrong direction)

Apparently this is not yet the solution to the puzzle, nor is the number 5 or 6 (the rotation used for the last (truth-)grid/the number found written in the last grid).
I'm not sure whether the hint 3+4+5=6 still needs to be used at this point..
Another way to go could be to start looking for the truth, since that's what the previous part actually suggests. Obviously the word truth was found in the fourth and last grid, which also has the word "six" in it, but we already discarded that possibility. Maybe we should go by the saying "the truth lies somewhere in the middle", leading us to the centre part of the grids/crossword, but that's just guessing and it has no logical basis.
The latest hint seems to suggest looking at the crossings of the crossword puzzle, which would be the letters H R T D O E A D I Y, but that doesn't really help either. Maybe we should focus just on the only real crossroads, the one where 3d and 6a cross around the letter O, but I have no idea what to do with that..

N.B., something that does add up nicely:

xifsf (ROT 1) --> where
ku (ROT 2) --> is
wkh (ROT 3) --> the
ywzym (ROT 5) --> truth
So the solution is: Where is the truth
Perhaps at this point the clue 3+4+5=6 needs to be used. Since the truth is in the grid with no. 6, we should add the Fibonacci numbers F(3)+F(4)+F(5)=1+2+3 = 6.. (this is assuming the Fibonacci sequence starts with a 0, otherwise we would have F(3)+F(4)+F(5)=2+3+5 = 10

xifsf (ROT 1) --> where
ku (ROT 2) --> is
wkh (ROT 3) --> the
ywzym (ROT 5) --> truth
So the solution is: Where is the truth

Update:

Apparently this is not yet the solution to the puzzle, nor is the number 5 or 6 (the rotation used for the last (truth-)grid/the number found written in the last grid).
I'm not sure whether the hint 3+4+5=6 still needs to be used at this point..
Another way to go could be to start looking for the truth, since that's what the previous part actually suggests. Obviously the word truth was found in the fourth and last grid, which also has the word "six" in it, but we already discarded that possibility. Maybe we should go by the saying "the truth lies somewhere in the middle", leading us to the centre part of the grids/crossword, but that's just guessing and it has no logical basis.
The latest hint seems to suggest looking at the crossings of the crossword puzzle, which would be the letters H R T D O E A D I Y, but that doesn't really help either. Maybe we should focus just on the only real crossroads, the one where 3d and 6a cross around the letter O, but I have no idea what to do with that..

N.B., something that does add up nicely:

F(6)+F(7)+F(8) = 8 + 13 + 21 = 42 :P

And the message reads:

This means:

Decoding the diagonal parts of the grids (xifsf, ku, wkh, ywzyh) with the corresponding numbers of the Fibonacci sequence (1,2,3,5) are numbers (3,4,5,6) of the Fibonacci sequence (these last numers as found by Tim earlier on).

And the message reads (based on the clue 6 1 4a 7 4d 9, 3 2 5 8):

The diagonals of the grids are:

 Xnoywt
 oIvwjh
 rdFsbr
 vsiSne
 hjqfFe

and

 Kruof
 qUmsh

and

 Werde
 wKvmp
 xjHij
 hlc f

and

 Ytdxo
 tWild
 ksZfe
 lomYw
 kwqoM

So we have to:

Decode these diagonal parts (xifsf, ku, wkh, ywzym) with ROT(X), X being the corresponding numbers that are found in the grid (as found by Tim earlier on).