B"H, 20 Shevat 5717.
Brooklyn, NY.
Harav v'chassid v'yerei shamayim ish emes noseh chen etc. Yaakov Meir
Shalom u'Vrachah!
Regarding what you wrote about the order of studying Tanya—it is proper to learn part three first, then part two, part one, and part four. As with all areas of Torah study, in the first rounds one should not get stuck on specific details or questions from elsewhere and similar matters. As our Sages said: "to review first and only later analyze."
Summary
The Rebbe teaches that when beginning to study Tanya (and all Torah), one should follow a specific order and not get bogged down by details or external questions at first—focus on overall understanding before deep analysis.