They are all in the same spot as they were in FE 3 with the exception of Pisces which now is not in a chest but is being held by a dragon that is ironically standing in the same place as the chest was. Here is a list of the locations I found them in the "Things I should know before I play this" thread
Taurus: Chapter 3 Enemy Ryuke (boss)
Gemini Chapter 5 Enemy Toras (boss)
Aquarius Chapter 6 recruit Samto
Cancer Chapter 6 Enemy Lang (boss)
Libra Chapter 6 Chest
Scorpio Chapter 7 Enemy Thief
Leo Chapter 8 Enemy Thief
Sagittarius Chapter 8 recruit Jeorge
Virgo Chapter 9 recruit Minerva
Capricorn Chapter 9 Chest
Aries Chapter 10 recruit Ellerean
Pisces Chapter 12 Enemy fire dragon