Answer: No, they are not equivalent
Why not? Well the 6x-3x portion simplifies to 3x. So the x terms are equivalent; however, the 4 and 8 in the first expression combine to 12, which is not equivalent to that 4 in the other expression.
In other words, 6x+4-3x+8 should simplify to 3x+12 and not 3x+4.