Hello, this is my first topic here 
I googled for it and searched here in this forum but it seems not to exist.
Is there a library or way to read/write to a SD card with ext4 filesystem (common in most Linux distributions)?
Best regards,
- mindbyte.
Really noone? 
There is not one specifically for the arduino however luckily for you most of the work has been done for you. I'd imagine that all you need to do is download the source code for ext4 found in linux, the kernel. Then replace the platform dependent functions with your own (as in functions to read and write raw sectors and such). I don't think it would be a terribly difficult thing to do.
I don't think it would be a terribly difficult thing to do.
How much program flash and SRAM do you think an ext4 implementation requires? You will never do it for an Uno.
ext4 is not a great choice for SD cards. SD cards are designed to be cheap with large flash erase blocks so there is a large overhead in rewriting file structures when you modify a file.
SD cards are designed assuming they will be formatted FAT with a file system layout specified by the SD association.
ext4 is a good file system but you can't take advantage of it with an SD card on an Arduino.
Porting ext4 code from linux is so hard that it has not even been ported to FreeBSD.
Why?
Simple answer: FreeBSD is not Linux, the implementations of ext3 and ext4 are so Linux specific that porting them to FreeBSD would be a major undertaking because of the differences in the internal implementations between FreeBSD and Linux.