Since you're running in cluster mode, your node may not be the driver. So it won't come at Spark UI of that but of some other node which yarn assigns as the driver.
For example you may be accessing Spark UI via http://localnode:4040, but in cluster mode it'll now be http://clusternode:4040. (because sc and driver are created there).
You can get this info through logs or resource manager UI.
1. Spark UI is available only during the lifetime of a SparkContext at <drivernode>:4040 by default. If you exit the context, it's gone.
2. The port number increments if you have multiple SparkContexts running. Suppose you created another spark context and ran some streaming job there, it would be accessible via <drivernode>:4041 now.