I heard that the F-4S is just and F-4J with electronic upgrades? Is there any exterior differences between the two?

Thanks

USMC0321

The S has slotted wing slats and a long wing fence.

Also, the S carries those long, thin antenna fairings that some restrofitted J's carried on the upper intake corners. There are some minor antenna details on the lower fuselage as well.

I started a conversion using the slats and actuator fairings from an Italeri 1/48 F-4G Wild Weasel kit, grafted onto a Monogram J. The issue of the Weasel I got also contained a small sprue of what appeared to be F-4S antennae along with a J/S nose (didn't need that).

HTH

f4s is actually f4j updated with slotted leading edges and stabilators+upgrade of weapons systems. For the antennae i will suggest you to check carefully wich aircraft you want to do because a lot of modification was done by units also and their specific request.
this kind of modification was not the same on all airframe . this due to the fact that there was different bunch of airframe.
some good reference : f4phantom.com F4 PHANTOM II THE STORY ( VERY NICE AND COMPLETE BOOK) DETAILS AND SCALE F4J /S AND F4 PHANTOM BY SQUADRON SIGNAL.