One of the main reasons to use fltk-1.4 is the wayland support, which means the tinycore fltk applets will work with a wayland compositor like labwc, weston, etc.
In order to reduce bloat, tinycore x86 does not use wayland. In addition fltk-1.4 is larger than fltk-1.3, so we decided to stay with fltk-1.3 for tc-16.x x86.