Hello,
same here – on WP 4.0 there is no map at all, only an altitude graph (can’t tell for WP 3.9, because I discovered this plugin only after upgrading to WP 4.0).
I tried the whole thing with Chrome, Safari and Firefox – no map.
But, when I load a map and GPS track with the OSM OpenStreetMap plugin, then the map is displayed also with WP GPX Maps. Somehow it feels, that something is not correctly “initialized”.
Also, everytime I enter the plugin settings, there are a lot of checkboxes, concerning the look of the map (width, height, style, …), unchecked, even if I activated them a minute ago, saved the settings, left the settings and entered the settings again.