i've heard a rav saying that every fourth step you do in Eretz Yisrael is considered a mitzva. does someone have a sources for this?
The Gemara Ketubot (111a) states that anyone who walks four amot (cubits) is assured a place in the World to Come:
א"ר ירמיה בר אבא א"ר יוחנן: כל המהלך ארבע אמות בארץ ישראל - מובטח לו שהוא בן העולם הבא
This would be roughly equivalent to four steps.
The K'nesset Hag'dolah (Orach Chaim 248) writes that he thinks he remembers the Shiltei Giborim writing that walking 4 amot is considered a mitzvah:
וכמדומה לי שראיתי בספר שלטי הגבורים, וכעת איני זוכר מקומו, דמהלך ארבע אמות בארץ ישראל מצוה קא עביד
The Gemara is referenced by the Rambam Hil. Melachim (5:11).
aish brings the sources
You shall possess the Land and dwell in it - Bamidbar 33:53
and the gemara in Ketubot 111a
Whoever walks four amot in the Land of Israel may rest assured that he will be a denizen of the World to Come
Ohr Somayach cites the Maharal explaining the reason
Because of the intrinsic Holiness of the Land of Israel, a person gains spiritual merit by merely walking here
See also aish for more details on the relevance of this mitzva today
The question, however, is whether this mitzvah is compulsory in our times when the Holy Temple is not standing. This is the basis of a dispute between two great Talmudic commentators, Maimonides and Nachmanides. A leading 20th century sage, Rabbi Moshe Feinstein, concludes that living in Israel is a "mitzvah kiyuma" – while it is a great mitzvah, there is no absolute obligation to do so.
See also here