Older key as PGP subkey? Currently I have a 2048-bit PGP key. Suppose I create a new 4096-bit key. Can I add that old key as a subkey of this new one?
Yes, PGP allows different-sized subkeys and subkeys are not derived of the main key, so it is possible to have the earlier key be a subkey of the new one.

How to do it is off topic here, but this should get you started if you use GnuPG.