You're not going to be able to accomplish this from xml only, you'll need some dynamic code to measure the width of the screen and then programtically set the width of each linearlayout (icon1, icon2 etc) to 1/4 of this width.
DisplayMetrics metrics = new DisplayMetrics();
getWindowManager().getDefaultDisplay().getMetrics(metrics);
LinearLayout.LayoutParams params = new LinearLayout.LayoutParams(metrics.widthPixels/4, LayoutParams.MATCH_PARENT);
LinearLayout icon1 = (LinearLayout) findViewById(R.id.icon1);
icon1.setLayoutParams(params);
//etc