You can reach Jyväskylä by train from Helsinki in even less than 3,5 hours, and from Tampere in 1,5 hours.
Check current offers on the VR website. Buy your tickets in the VR web shop here. You can also buy your train ticket in the vending machine at the Jyväskylä Travel Centre. You can find the VR mobile app at App Store or in Google Play store.
If you fly to Helsinki, you can get on the Jyväskylä train connecting easily directly from the airport with just one change of trains. By train, you can also travel to Hankasalmi, Petäjävesi, Keuruu or Jämsä, for example.