Practice an alternating use of 1st and second finger e-f-e-f-ef on d string. Do it also in different string like on g string which u will be pressing a-Bb-a-Bb etc. also in a and e string.
For your fourth finger, just practice about the same. Do alternating 3rd and fourth. This will help strenghening your fingers and more practice will give you a muscle memory which in time you will get use to it.