First: You shoeld download 4.8.1 now.
In both version 4.8.0 as well as 5.3.0 session sign-out is delayed, which could cause issues with the licensing mechanism and/or instability of the application. This is especially relevant when issuing (a lot of) web service calls in a short time frame. All applications, and especially applications exhibiting such behavior, are strongly recommended to upgrade to either 4.8.1 or 5.3.1 to eliminate the risk of this bug affecting your application.
Afther that, you can set the JDK used by the modeler in Edit->Preferences.
I looked in the runtime folder of 4.8.1. the java version seems to be (build 1.6.0_45)
assuming that the same version is used for 4.8.0