"Should I put all vertex data to a single VBO?"
Whereever possible, yes. It means the whole vertex will be pulled into a cache at the same time. By having multiple VBOs, you're using multiple memory units which means multiple caches need to be loaded. Each will contain the vertex data for more vertices, but that's not necessarily useful; if you use the coords for a vertex, you'll use the normal data and probably quite soon, so the memory effort in loading it is useful.
If you have multiple memory units loading many verts into their cache, their effort may well be wasted -- touching vertex V implies nothing about when or even whether you will need the data for vertex V+1.
"If a group has, say 4000 polygons (which could happen for the player models), is it advisable to call glDrawElements once for the whole chunk, or should I cut it into pieces?"
Do the whole thing. Reason; let the driver do the optimisation work. It knows what shape the hardware is, and you don't. Don't try and second guess it unless you have a known crap driver you're trying to work round. Some drivers, for example, may take the min/max vert index and transform everything in that range and then bin the unused values. If you unchunk the data, they'll obviously waste more time on unused nodes. For the same sorts of reasons, try and make sure all the verts in a chunk are adjacent in your VBO.
You're also likely to transform verticies on the split boundaries more than once, whereas as one draw, they'll likely only be done once.