Fsolve solve for numeric when I put X as an parameter in one of the equation it get an error

restart;
y1 := 2*a*b^2-b+c-d-1 = X;
y2 := a^4+b^3-b^2+c-d+2 = 0;
y3 := a*b^2-b+c-d-3 = 0;
y4 := a^3*b^2-b+c-d^3+4 = 0;
fsolve({y1, y2, y3, y4},{a,b,c,d})

Error, (in fsolve) X is in the equation, and is not solved for

Hi

I started my university project 5 months ago and working with maple from that time.

I used maple help to solve my problem but for this one I could not.

I have equation system with 4 equations and variables are for example x y z w .

The problem is I  have  xy  x^2  x^3  yw   in my equations so maple with solve could not solve that.

On the other hand,the equation are parametric and non numeric...