Personally saying you should avoid biking in April , if possible start in May. Roads are open from Shimla side via Narkanda. From Manali side Rohtang pass would be closed till mid of June(depends on BRO).
Good to go... April will have some unique challenges..like snow started melting .that means..all the water crossing will have water. Roads will be ok but it will be very cold. that's amazing...