How file system block size works?

All Linux file systems have 4kb block size. Let's say I have 10mb of hard disk storage. That means I have 2560 blocks available and let's say I copied 2560 files each having 1kb of size. Each 1 kb block will occupy 1 block though it is not filling entire block.

So my entire disk is now filled but still I have 2560x3kb of free space. If I want to store another file of say 1mb will the file system allow me to store? Will it write in the free space left in the individual blocks? Is there any concept addressing this problem?

I would appreciate some clarification. Thanks in advance.