I think 4 years is a long time for anyone to be dating without an engagement or committed plan. I don’t know that much about their culture or religion,but I think it is possible that they like you and also possible that you may never be acceptable as a marriage/life partner for him. Not that you are not a great person or good for him,but you are not “their kind”.
You need to find out your BFs intentions. Would he want to be with/marry you even if they don’t give “their Blessing”? He should be dealing with his mother and telling you what the stumbling blocks are there. Would she “allow” you to visit if you two were engaged? He should always be willing to take your side over her i he is serious.
You sound very passive and IMO you need to get more info on his intentions,ideas and plans before you spend more time in limbo. You also need to know if and how your position would change if you did marry. How would you be expected to “act” etc.