Not sure where you saw the BK1000 designation - I haven't seen that one.
My understanding of Boker's current model designations are the prefix (11 for folding knife and 12 for fixed blades), followed by the pattern number (in this case 1000 for the "fellow" pattern), then a suffix describing the handle material. (example: JRB, for jigged red bone)
My understanding - and I may be wrong - the 1000 pattern is the "fellow" pattern. The 4100 is a fellow pattern with stag handles.
Kind of reminds me of the old SAT statements: "All 4100s are 1000s, but not all 1000s are 4100s."