Looking for speakers for my 4W SET amp

I would suggest calling Louis. With your music preference I am guessing he would suggest the alnico. I do not use a sub(s) with my 3XRS HO (:

isochronism,  I have an Elekit TU-8200 kit.  To my ears, it sounds best in triode mode.  Other modes are ultra linear and pentode giving 8W per channel but they don't sound as good as the 4W in triode mode

My Heresy IIIs required a sub or two (which I already owned), but add those and the Heresy IIIs are astonishingly coherent great sounding things, and relatively inexpensive.

There are a lot of choices.  A search on Audiogon will reveal a slew of threads.  My short-ish go-to list of SET friendly speakers include:
- Klipsch Heritage.  Heresy to Khorn, depending on your room and your budget
- Omega.  All good.  Speak to Louis for specific recommendations
- Coherent.  Great speakers from the great white north.  I own a pair of 8s and they are very good but I hear the 12s and 15s are fantastic.
- Diatone.  This is Japan's JBL.  A wide range of speakers in various sizes and configurations.  I don't think these guys ever made a bad speaker.  In the U.S. most models are branded as Mitsubishi (but not all Mitsubishi speakers are Diatone).
- Altec 17 and 19.  Big and bigger.  Other Altec models as well
- McIntosh ML-1C.  Don't laugh, these can sound quite good (in a vintage way) IF they are recapped and paired with the MQ101 equalizer.

4W is tough - depending on your space, how loud you like to listen, your choice of music, and even (perhaps) the length of speaker cables. It is right on the cusp.