In the situation you describe, with a vector of ones in the first column of your X matrix corresponding to the parameter that equals zero, the zero value means that the offset (analogous to the y-intercept b in the linear equation y=m*x+b) is zero. The equation is essentially y=m*x.
Stated more simply, it means that the hyperplane that fits your data passes through the origin. Your Y data are likely described by x1, x2, and x3 only. However I suggest you use regress (Statistics Toolbox) to get the confidence intervals on the parameters (and optionally other statistics on the fit) before you draw any firm conclusions.
