What is the last digit of $14 \uparrow\uparrow 14$

See Knuth's up-arrow notation for details of the terminology.

4
6
2
8

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The last digit of ${{{{14^{14}}^{14}}^{14}}^{\cdots}}$ is $\boxed{6}$ because $14^{1} = 4, 14^{2} = 6, 14^{3} = 4, 14^{4} = 6, \cdots$ . Thus, the answer must be $\boxed{6}$ .