Busy beaver shift function $BB(4)$

Rules and space-time diagram of the 4-state busy beaver that runs longest

The maximum number of steps made by a $4$-state, 2-symbol Turing machine that halts when started on a blank tape.

Value: $107$

Updates from the literature

Lower bound: $107$
Reference unknown
[via Sequence A060843 in the OEIS]
1983-04 Upper bound: $107$
Brady, A. H. (1983). The determination of the value of Rado's noncomputable function Σ(k) for four-state Turing machines. Mathematics of Computation, 40(162), 647-665.
[via Sequence A060843 in the OEIS]