There is a very basic reason why your 4GB card is not being recognized by your phone. All the Nokias only read and write FAT16 file systems on the expandable memory media. Now, FAT16 file system has a limitation of 2GB on a single volume. This is the bottom line, technical reason why a 4GB card won't work in any of the Nokia phones and no firmware upgrades will change that. The only thing that might be any kind of possibility here to circumvent that limitation would be to format the card with 2 separate 2GB partitions. However, I seriously doubt it that the phones will actually support what would pretty much amount to having two separate memory cards. I don't think that the Series60 supports that and I have doubts that the underlying Symbian OS itself will actually support that. I suspect that the phone would either not boot with a 4GB card with two partitions on it or in the best case, would only see one of the two partitions. For one thing, I don't think that the phone would know what to do witht the second partition and the OS and/or Series60 GUI doesn't even know that it should assign a different drive letter to it if it is present. Chances here are slim to none. Still worth trying though.
I'm actually glad to see that this has become a question this quickly. It was only a matter of time before the 4GB miniSD cards became available and this issue became a question. So, you Erkk, have the unique opportunity here to become the first person on HoFo to try and format your 4GB card with two 2GB FAT16 partitions and stick it in your phone and tell us all what happens. I really don't see how it could damage anything, your phone will probably just not see the card, not see the second partition, or just not boot at all with the card formatted that way. Let us know what happens if you decide to do this, or either sell the card of find some other use for it.
Cheers and good luck....