Briefly, the Weston UM3X is better than ER4P when used with a portable source. As for ER4S, I understand that UM3X is a version suitable for DAP's without the use of external amplification, while the ER4S is a "professional" version and need a good source and a powerful amplifier. Ultimately, the UM3X and Ety ER4S do not share the same sound signature, however, share a number of characteristics that make them somehow, competitors.
The neutral sound signature, the perfect tone, great separation between instruments and high detail are the basic features found in both headphones.
I love both!