the ONLY way to get from T4 to T8 other than an airport bus is the airtrain, which is free inside the airport. in T4 the station is located "within" the terminal building, after customs. everyone who enters the US from another country must clear immigration and customs. The ride from T4 to T8 should be less than 5 minutes. Arrival time at your departure gate in T8 is probably around 30 minutes including check-in and security. provided your flight is not at one of the busiest times morning or late afternoon, you should find all transitions smooth, although there is probably about .75km of walking involved, all told.
transiting in JFK