macro solver changing cells

Hi, here is my problem:
Capture.PNG

I need a macro linked to solver so when I click on one button, it will converge input value to target value by changing cells in two cases
case 1: seek a solution by changing A,B,C,D, constraint is 100% in cell C4
case 2: seek a solution by only changing C & D, keep A & B fixed. constraint is 50% in cell C4 (sum of C & D).
The problem is the input value is the sumproduct of A,B,C,D so how can I let 2 variables on float while the other 2 fixed?

here is my macro:

Please Login or Register  to view this content.

one solution I think of is creating a msgbox pop out to prompt me enter the cells to be changed, like the original solver which I can select the changing cells but I can I do that?
is there any better solution?

Thanks a lot!!!