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Teensy 4.1 can be upgraded with up to 16 Megabytes of PSRAM. 

I have found a compatible APS6404L 3SQR 64Mbit PSRAM chip on Aliexpress. So I thought it was worth trying and ordered it there for 1 euro 30 cent including shipment. The order was delivered within 2 weeks. 

Teensy 4.1 has two expansion sockets. You can install two PSRAM chips (64Mbit each) or one PSRAM chip and one flash chip. I chose to solder one PSRAM chip and later I might add another PSRAM or a Flash chip. I still haven't made up my mind on what is the best solution.

First I have added some solder flux gel on top of the pads.

For some stability I have glued a sticky tag on top of the Teensy board.

Then I gently placed the chip using needle nose tweezers.

To solder I have used a thin 0.5mm solder wire from STANNOL.

Using a needle tip on my soldering iron it was easy to solder all of the pads.

After soldering it is a good idea to clean all of the remaining flux using isopropyl alcohol

To check if the upgrade went OK you can download an memory test sketch and upload it to your Teensy 4.1 using Arduino IDE. The code first checks if the external_psram_size variable is set, which indicates that the memory was expanded and then fills the memory with test patterns and  tries to read it back.

The output is shown inside the Serial Monitor. Luckily, in my case, all memory tests passed successfully!