First off, you mention the weight of the FED 4. Although there's some variation between models, all the usual FSUs are metal-bodied and they are all relatively heavy. Also, you have a light meter and slow speeds on your FED 4 that you'll lose with the other suggested models.
The FED/Zorki 1 are smaller but not that much lighter. With a collapsible lens, they do fit a large ish pocket though, unlike the FED 4. Downsides are squinty VF/RF (but not unusable by any means) and lack of slower speeds.
The Zorki 5 and 6 are smaller than your FED 4 and have much nicer VFs. The FED 2 does have a reputation for reliability and it has the wide-base RF (as do the Z-5/Z-6).
If you're really wanting to go a fair bit smaller, I'd opt for a late Zorki 1. Just don't expect it to be half the weight and take a meter (if you use one).
Zorki: 1e (x2), 2C, 3M, 4, 4K, 5, 6
FED: NKVD, 1g, 2e, 3a, 3b, 4L (x2), 5B, 5C
Kiev 4, 4A
Yashica Minister III
Zenith C, Zenit C, Zenit E
Minolta XG M, Z1
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