Identification 4 pins crossing High-low voltage boundry, marked ABS10, 2 pins marked + -
It's a bog-standard bridge rectifier. It converts the AC into (pulsating) DC which is smoothed by the cap and fed to the rest of the converter.

More specifically, it's an ABS-10 bridge rectifier, available from several manufacturers, rated for a max RMS voltage of 700V and max average forward rectified current of 0.8A.

Here's a datasheet from Multicomp.