Cathy King, "The fourth century coinage," in L'"inflazione" nel quarto
secolo D.C., Instituto Italiano di Numismatica, Rome 1993, pages 1-87
(especially pages 30-33).
These paragraphs summarize her work.
The copper coinage after 364 was just that, copper, with no intentionally added silver. Prior to 364 coins like the "bull" of Julian had a non-negligible amount of silver (2%), although his AE3 had effectively no silver.
Kent thought that at the start of the reign of Valentinian the denominations continued those of Julian -- an AE1 of about 36 to the pound and an AE3 of about 96 to the pound. The smaller one was soon replaced by a coin struck 132 to the pound (Kent and Bastien agree on this). This was the lowest denomination until 378. After Theodosius became emperor a heavier denomination was issued c. 381, perhaps at 60 to the pound (AE2). It was issued until c. 388 in the West and c. 395 in the East.
The AE4 was introduced at Rome c.379. Its weight was reduced about the same time the Theodosian AE2 was introduced. Between 381 and 408 the weights of the small pieces probably decline twice, but the weights are too erratic to be sure what standard was intended.