Using a Multi-variable regression fit with constrained positive valued coefficents how do I get an error for my coefficents?

Just wanted to know if anyone could recommend a nice function in Matlab I should use which will return the coefficients of a multi variable regression fit e.g. return values of A,B,C and D for the following case

yi = Ax1 + Bx2 + Cx3 + Dx4

where yi,x1,x2,x3 and x4 are all arrays of the same size 1 by n.

I have used things like regression,robustregression,lsqlin and lscov commands. The problem is that A,B,C and D must all be positive values and I also need to know the error on each of the coefficents returned.

Thanks in advance

Chris