LR4 Catalogue file - how do I move it from my "My Pictures" folder without breaking everything?

You can move the Catalog with no great consequence to LR, provided you move it together with its associated previews folder. Only when you next start LR, it may prompt you that it can't find the last-opened Catalog, in which case you would just tell it what to do - browse to the moved Catalog in the SSD.

http://www.computer-darkroom.com/blog/will-an-ssd-improve-adobe-lightroom-performance/

As for moving the image files, which is a completely SEPARATE issue, there are two options: do so inside LR, or do so outside LR and then re-connect using a single parent folder. Say you have a "pictures" folder inside your new external drive: you might get Lr to show the My Pictures folder of your current setup by using "show parent folder" as required; then re-address this onto the Pictures folder in the external drive.

http://tv.adobe.com/watch/learn-lightroom-4/import-moving-folders-around-after-the-fact/

Make sure you have set a locked drive letter for this external drive; if that drive letter wanders around depending what other USB bits and pieces are connected, LR will keep complaining about missing (more accurately, not where it used to be) data.

RP