Visiting St Petersburg in April can be very rewarding. April weather is much milder than for the six preceding months of winter. Spring is very much in the air, and the last of the ice and snow has disappeared by the end of the month. There are other advantages too.
The main one is that you’ll be able to visit the Hermitage, museums, churches and other attractions in relative peace. The visitor season gradually gets busier throughout May and June and is at its peak in July and August, where you can find yourself jostling for space at major sights. However, visiting St. Petersburg in April means that you’ll have plenty of time and space to see everything you want to see, and won’t feel herded around with a crowd.