4GB should be fine...
Did you load the software to mount a Fat32 filesystem on your Amiga?
Might be it doesn't like the card...
Also, what kind or RAM expansion/accelerator does your Amiga have, if any?
Some don't cooperate with PCMCIA too well...
Also, you might need "cardreset and/or cardpatch" in your startup sequence..
(You can find those on Aminet)
There is a whole thread on getting PCMCIA CF working:
https://eab.abime.net/showthread.php?t=30757
It's like 23 pages long, so something to search and/or post there to get the experts assistance... ;-)