Get your urologist or GP to prescribe you an anti-spasmodic medication. I use Oxybutynin, as it works best for me, but there are numerous others on the market.
Are you sure your stone is 4cm??? That's awfully large for a kidney-stone. There are 'staghorn' stones that form inside the kidney itself and fill the cavities of the kidney (the 'poles' of the kidney) that may get that large, but the chances of having a 4cm stone in your bladder are slim.
I formed a 17mm stone, and when my doctor pulled it out he was impressed by the size, so I feel quite sure that you meant 4mm, and not 4cm... is that correct?