Assuming that c is a "conversion factor" to convert between space and time,
Then, in 4-vector, we have x_1 through x_3, and t, where, x/c = t
x/c = t, (where t = time, c= lightspeed, x = spatial dimension)
If we do what we did to space to get time, to momentum,
p/c = m*v/c = m (x/t) / c =...