inexpensive dualSIM phone
 

ekit V4D active dual SIM quad-band
The above phone is light weight and simple. If you want a new, simple dualSIM phone, you might consider this. $39

Also, it takes very standard Nokia batteries: BL-5C

another thing
 

While it does allow you to manually select networks; while having two TRU SIMs in the phone and I try to change the default [!] network from AT&T to T-Mobile, after choosing T-Mobile it always go [for both SIMs] back to AT&T. Not a big issue but a quirk.

yet another thing
 

When I put a Spot Mobile SIM in slot a it says: Vstream and the second SIM, a Tru says: Vstream_Tru. Of course, Vstream is a blast from the past and is now T-Mobile.

When the Tru SIM says AT&T, it says: AT&T_Tru.

The operator names are usually a list inside the phone. Thats why you will see operator codes instead of names (or old names) when using older phones...

new phone////old list
 

I am fairly confident that the phone is less than two years old. The 'list' is just ancient.

I bought this phone last year (from the same seller linked here) and it works great as a cheap dual-sim.

Are there any quad-SIM phones available besides the Anycool S320 and the iPro FX3 Pro? Does anyone have any experience with these?

Yes. Look on ebay and search for 'quad-SIM'. I got one, but it died. Slots 3 & 4 were not as full function as slots 1 & 2. This was 'about' a year ago. i would think they are better and cheaper now. I decided to go with dual-SIM phones. I have bought 4 in the last 60 days. [Two ekit V4D and two BLU Samba Q51 {qwerty}.]

I did - these were the two models available, with the S320 being a lot more common (although I would prefer the other one). I've ordered an S320 and will let you know what I think of it when it arrives.

I would be curious about if they have improved in the last year.