Need different databases to different blogs in one WordPress multisite

Hi,

i have a WordPress multisite for making around 8000 blogs. The problem i have is the size of the database. Hoy can i get, with 4 databases, to assign 2000 blogs to each database? I have tried hyperdb and shardb but i don′t manage to make it work. Is there any simple way to do it?

Thanks so much,

regards!