Rank of a matrix built from four row vectors

Let $M=\sum_{i=1}^4X_iX_i^T$, where $X_1^T=[1 -1 1 0]$, $X_2^T=[1 1 0 1]$, $X_3^T=[1 3 1 0]$ and $X_4^T=[1 1 1 0]$. Then the rank of $M$ equals ____________