Comment # 13 on bug 101325 from Julien Isorce

apitrace dump:

94 glShaderSource(shader = 2, count = 2, string = {"#version 150

#define PLATFORM_USES_ES2 0

#define PLATFORM_LINUX 1
", "
#if PLATFORM_USES_ES2
precision highp float;
#else
// #version 120 at the beginning is added in
FSlateOpenGLShader::CompileShader()
#extension GL_EXT_gpu_shader4 : enable
#endif

varying vec2 textureCoordinate;

uniform sampler2D SplashTexture;

void main() 
{
        // OpenGL has 0,0 the \"math\" way
        vec2 tc = vec2(textureCoordinate.s, 1.0-textureCoordinate.t);

        gl_FragColor = texture2D(SplashTexture, tc);

}"}, length = NULL)

So it does not look that GL_EXT_gpu_shader4 is really used.
Also I wonder if you can configure UE4Editor to use egl/gles2 and see if it
still hangs.