Is it possible for me to go back to the frosting and do it my self
It may be possible to frost over what you have, but remember that you have both frosted and tinted hair even though the frosted hair doesn't show. If you frost over the old frosting, you might "melt" those pieces of hair. ( melt = anywhere from turning mushy to breaking off)
Is 4 weeks okay?
To be safe, the old frosting should be grown out and cut off. Your hair grows approximately 1/2 inch per month.
Sorry to be such a spoil-sport. I'd have to see (and touch) your hair to be sure that it would be strong enough. <img src="/images/graemlins/frown.gif" alt="" />