Lightroom 3 lightroom 4

Question

If I have a lightroom 3 catalog can I open it in lightroom 4 or will I use my settings

Answer 1

I want to take the archive catalog OFF of LR 3.2, put it on the Air, running 4.4.1 - then ADD metadata and keywords to the catalog images while on the Air and THEN re-install/import/export/etc the catalog back to the main tower LR 3.2 with out starting a huge problem of version/upgrades/etc. That way another person can be using the tower to scan while I do keywords
.If you import a LR3 catalog into LR4, any difference in "process version" for the Develop settings etc, is handled by LR retaining and respecting however the images came in, unless/until you say different. So the LR version should not be an issue, provided you are content to transfer your keywording etc via externally written metadata, and not via a Catalog (which is not, as you say, backwards compatible). So that constrains your use of Collections and of Virtual Copies, which cannot transfer that way.
One possible workflow - just for explanation purposes, to be adjusted as the circumstances dictate - the images can be scanned and saved to disk but not yet imported to LR3, and then put somewhere visible to your LR4. Then imported into your LR4 catalog. Setting the images to a LR3-compatible process version within LR4, will ensure backward compatibility of any Develop adjustments you may do there. But keywording etc should not care about that AFAIK.
Then write metadata to the files (Command+S or set your Catalog settings to do this automatically as you go). These images with that accompanying metadata, then need to be put somewhere visible to the LR3 catalog, and imported there, at which point all that metadata will automatically get used.
Alternatively, if the images have to be imported to LR3 first, then (with all the same comments about Process Version) - after bringing into LR4, and writing metadata back to the files from LR4, and getting the updated image files and metadata back where the LR3 catalog originally saw them - the LR3 catalog can then be told to "read metadata from file" so as to bring in those updates. Important note: this is all-or-nothing and will obliterate all work done on those images, in LR3, in the meanwhile. So some careful working rules are needed - or there may be ways including e.g. Snapshots to get around even this.
I hope this helps to, at least, suggest a workable solution - or raise further specific questions - regards, RP