Finally I figured out how to do it. Thanks Ole, your help led me to this solution.
So this is the working code:
Initializing the buttons and the button group:
tb_one = (ToggleButton) findViewById(R.id.instant_tb_one);
tb_one.setOnClickListener(this);
tb_two = (ToggleButton) findViewById(R.id.instant_tb_two);
tb_two.setOnClickListener(this);
tb_three = (ToggleButton) findViewById(R.id.instant_tb_three);
tb_three.setOnClickListener(this);
tb_four = (ToggleButton) findViewById(R.id.instant_tb_four);
tb_four.setOnClickListener(this);
rg_modes = (RadioGroup) findViewById(R.id.instant_toggleGroup_mode);
rg_modes.setOnCheckedChangeListener(ToggleListener);
rg_modes.clearCheck();
rg_modes.check(tb_one.getId());
The onClick Handler:
if (v == tb_one|| v == tb_two|| v == tb_three|| v == tb_four) {
rg_modes.clearCheck();
rg_modes.check(v.getId());
}