x1=j1=t1 represents the numeric base of x2=j2 evaluating to x3; x2 is in base x1 + 1 which equals x4; x1 is the largest possible single digit in the representation used by x2.
x2 does not have to include the largest possible single digit; this would instead specify what the base of x2 is. The base has x4 digits. See also: ju'u'e, ju'u, ju'umcu.