This is not a question, as I have already figured out the fix. I am posting this to share my findings.
My original problem:
Laptop with Windows 7 64Bit + PCLink v3.4 and G4 (want to upgrade the LEM G3 to G4 firmware). Problem is that PCLink does not come with 64bit drivers, and the drivers it has are unsigned.
At first I disabled signing enforcement but that did not help as the driver didn't work (I guess it is 32bit specific).
As everyone aware I am very anti-windows, so I am not going to go and acquire Windows 7 32bit just for that purpose (I believe it is wrong approach).
In desperation I decided to pull apart the ECU to see what the internals are and specifically what USB to UART bridge they used.
It turned out they used Silicon Labs CP2102 (which for some reason was on opposite side of the board to the USB socket). I googled and found Win7 signed driver (which included support for 64bit): http://www.silabs.com/Support%20Documen ... ista_7.exe
So I downloaded that and installed, then went to Device Manager and updated driver to silabs one. Immediately I was able to connect to ECU.
One thing I noticed is how little of hardware is on that ECU. I am a bit disappointed how little hardware you get for money you pay. Especially when I found out that they use some shitty 16bit 40MHz microcontroller with 12kB of RAM. Also for some reason they use two separate and different ATMEL FLASH ROMs (perhaps cheaper one for permanent firmware, and more expensive for user data).
Basically the amount of hardware is around $100(best deal)-$200(retail) tops. If you compare this to the video card (or Atom mainboard), it seems that there is a lot of markup on that (R&D?).