LEAD TIDBIT:
Mitzva Number 4
We know by an ancient tradition that the Torah has 613 mitzvot. Exactly what the 613 mitzvot are is a very disputed issue. Rambam’s Sefer HaMitzvot and the Sefer HaChinuch are probably the two best known works on the 613 mitzvot, but there are many others. Ramban is a major antagonist of the Rambam in the counting of the mitzvot. He challenges over 70 of the Rambam’s mitzvot - not whether something is part of the body of mitzvot and halacha, but how and what are to be numbered among the 613.

Many printed editions of Sefer HaMitzvot of the Rambam include the challenges and comments of Ramban.

It is not sufficient for Ramban to claim that a particular mitzva on Rambam’s list should not be counted, separately or at all. He cannot leave the list short of 613. Therefore, those editions of Rambam’s counting that contain Ramban’s comments also contain a list of those mitzvot that Ramban counts among the 613, to bring his list back up to that number, so to speak.

What has become known as “The fourth mitzva”, because of its position on the Ramban’s list, is Mitzvat Yishuv Eretz Yisrael, the command to accept G-d’s offer of the Land and to live in it (having done what it takes to accomplish that goal).

Ramban’s view is that the mitzva applies today, just as it has always applied, and that one is duty-bound to live in Eretz Yisrael.

Many argue that this view is shared by Rambam, even though he did not number the mitzva among Taryag (613). Rambam considered himself a sinner by not living in Eretz Yisrael. He quotes the Gemara, that one should live in Eretz Yisrael even in a city the majority of whose inhabitants are idol worshipers, rather than live in Chutz LaAretz, in a city with a Jewish majority.
It is an undisputed halachic ruling that a husband or wife can insist that their spouse come/go on Aliya with them, and if the spouse refuses, he or she is penalized. The existence of this ruling is considered proof that there is a mitzva to live in Eretz Yisrael... and it’s a big one!

In addition to the mitzva to live in Israel, there is also the fact that many mitzvot apply only in Israel. Living here facilitates the fulfillment of other mitzvot, even without a Beit HaMikdash yet.

“It’s difficult.” “I have elderly parents in the States, how can I leave them?” “My profession is not good for Israel - how will I make a living?” There are many reasons for not living in Israel. Some of them are “good” reasons; others are “just teirutzim”. All that is besides the point here.

The point is that it is a mitzva to live in Eretz Yisrael. (Whether it is an obligation-type mitzva or a fulfillment-type is also besides the point.) It also is a wonderful and special place to live. And it is obvious from numerous p’sukim in the Torah that G-d wants His people to be in Eretz Yisrael.
There’s a lot more to say, but that’s what future TTs are for.