Good food for thought. I've read about voltage drop, it's not something I'm well versed in.
I decided to order a unit I put up a link for above.
DC-DC Adjustable Buck Voltage Converter Stabilizer Step Down Voltage Reducer | eBay
For $11 seems a reasonable gamble. The part looks more involved in what you'd think you could get for that sum.
My thought is to eventually have a higher quality transformer for all four. If the units themselves, the LEDs in total, are high enough quality to last significantly longer than the transformers that came with them could be a reasonable investment. Would be good to know about zener diodes however. I'll look into it. I'll see if I can determine the resistance of each globe. Would be a good learning process.
This client has a charming but old house and they give me a lot of work. Plus I really like them. He's 63, she's 58 and they are both gorgeous. More than skin deep in this case. They work at giving good party for their clients, the cool backyard is a big part of it.