For matrices of same dimension $M, N$ and scalar $c$, which one of these properties DOES NOT ALWAYS hold?
1. $(M^{T})^{T} = M$
2. $(cM)^{T} = c(M)^{T}$
3. $(M+N)^{T} = M^{T} + N^{T}$
4. $MN = NM$