You should use the latest Mendix version which is 4.5.0. The Modeler will install the necessary JDK for you if you don't have it.
Oracle's versioning is confusing, but 1.6 is the same as 6. This version is indeed getting old but very stable. Our cloud still runs on this version and when building deployment archives with a newer JDK the application won't start, that's why the Modeler still uses 1.6
Are you sure you can't choose version 3.3.8 on the forum? I added it the moment it came out.