Since exp(a-b)=exp(a)/exp(b) you will need the distribution of a-b and then the distribution of exp(a-b).
If a2, b2, c2, and d2 are approximately normal then exp(a-b) will be approximately log-normal. If you have covariance between parameters then the covariance matrix of variables (i.e. covariance of each pair of variables) needs to be taken into account. You can estimate co-variance of two variables using standard techniques.
Note that if the estimators you are using are maximum likelihood ones, then you can use results regarding estimators of functions of parameters as well as asymptotic results regarding normal approximations.