One of our reasons for developing 4G Mission Theory was that immigration causes havoc on the definitions of people groups. For example, if an immigrant community is significantly touched by a movement of the gospel but there is no parallel movement back in their homeland can that be seen as impacting their "unreached" status? Or, what if the majority of the Christians emigrate, are those left behind more "unreached" than they used to be?
There are other funny things that happen when people immigrate. They define themselves differently. I myself grew up in Canada but immigrated to Australia the year after marrying my Australian husband. During our years as a family in Russia when I was asked where I was from I began to answer "Australia", especially after we had kids, to try to keep it simple for them. Now that we are back in Australia people still ask me where I am from my answer is once again Canada. And when I'm in Canada? Yeah, who knows.
So, if you ask an immigrate their cultural heritage they might say "Filipino". But, among the Filipino community, or while back in the Philippines they are distinctly Cebuano. (or one of hundreds of other minority people groups of the Philippines.)
Sometimes immigration erases boundries that were more significant "back home" and communities are formed of people who would otherwise be multiple people groups.
But sometimes it can have the opposite effect. Immigration era becomes important. Ukrainians who immigrated 3-4 generations ago are not likely to have much in common with new immigrants from Ukraine. They might be sympathetic and helpful, but in terms of reaching a people group with the gospel they are two seperate people groups. The Ukraine has change a lot in that amount of time.
4G Mission Theory measures relational distance rather than geographical distance. So, for the most part immigration changes nothing in the 4G way of looking at the world. A 3G people group, take the Bengali for example, are 3G for the average Aussie, whether we are talking about the Bengali who live in India, the Bengali of Bangladesh or the Bengali in Oakleigh (Melbourne). See? Isn't 4G Theory helpful ;)
4G Mission Theory is still in development. Since I've got your ear I'll let you in on one thing I haven't nailed down yet. One of the defining differences between 3G and 4G people groups is the degree to which they relate to global civilisation. 3G people groups are culturally different to us, but still have universities and airports for example. 4G people groups in many corners of the earth may not have passports, or access to formal education. Generally speaking - 3G people groups immigrate all over the world. 4G people groups don't. Except, they can be chased out as refugees. So, the Rohingya, the Hazara - they are oral minorities, semi-nomadic who live far from the centres of civilisation in their home countries without electricity or modern medicine and educating their children by their own means. They would be 4G people groups. But here they are suddenly settled in units and houses in Springvale and Narre Warren. So, are they still 4G? or are they now 3G having lost most of the distinctions of 4G life along with their homes and everything else when they became refugees?
Immigration is tough. It changes identity so much! But becoming a refugee is so much more! Maybe 4G Theory can be part of helping us understand how those two things are different?