PGP key: length, multiple identities, smart card

Question

I am new to GnuPG. I have generated a key in with Enigmail in Thunderbird so far, which is also available through Seahorse now.

1. I generated a key with 2048bit. However, I read that 4096 is now recommended for some purposes. Since my key is not distributed yet, I can generated a new one at any time. Should I generate one with 4096bit? Are there any disadvantages? Incompatibilities?
2. I noticed that I can add multiple email-addresses to one key. Is this advised? I assume this might be a privacy problem if I do not want to have my business and private email-addresses linked.
3. I have a smartcard which has a signature subkey, encryption subkey, authentication subkey. What is a subkey? How does the smartcard interact with GnuPG? Is there a key pre-installed on the card? How can I be sure that this key exists only once? Can I derive a key from the card so that I can read my emails also on computers where I do not have a card reader? What happens if the card breaks/gets lost? Can I make a backup of the keys on the card?
4. Can I use my PGP-key to encrypt my home directory? Would this make sense?

Thanks!

Answer 1

1. 4K keys are a little bit larger and need some more processing power for creation and signing work.
2. Only you can decide that. If you don't want to link business and private keys, create two of them. Also read this answer on security.se to a similar question.
3. The FSFE fellowship smartcard contains already contains a key. All your questions to the smartcard are in detail answered in the FSFE wiki apart from the one whether a key only exists once: You cannot know apart from checking keyservers, but you will never know of a key that is not uploaded to keyservers (or used otherwise).
4. I know of Symantec Symantec Drive Encryption which uses OpenPGP keys, but is not free/libre software in case that matters to you.