Yah, titan is right, you ignore them. If they are 5k below you and you want to jump them, follow them while using throttle to position yourself, they will turn below you and you position yourself in a low e state (because for a 109 and especially a K4, you don't don't want excess e if you have alt) and do your roll over with little or no throttle, saddle/shoot and then gently throttle up for the zoom for a quick high yoyo/barrel roll to keep the pressure on.
Personally I prefer to get under them and take the shot on the up since if they break with a split S, you can hammer it easily and then it is a saddle for you, likewise if they go up or break to the side, you have the extra e to manage throttle and have gravity working against you. Personally I like the 109 in the vert going up more then in the dive and like the merge, will always try to get under (but more e) then the opponent. This is the exact opposite with in the 190 where i prefer coming down on someone.
Ultimately speaking, the 109 is not a good BnZer really, for the aforementioned reasons and because it lacks the snapshot firepower. Yes yes the tater is a one shot kill, but it is best shot from very close ranges with high profiles - and these kinds of situations are usually in the up hill pert of a vert fight. In the down, you just don't have time to really be accurate - whereas the 190 can put out a fair bit of ammo and start shooting further out. So i suppose the crux is: if you want to keep with the BnZ style of fighting, I recommend a different plane that is more suited. Mind you anyone can BnZ in any plane - but some are more suited to the task, and like the KI84, I prefer to start with an alt disadvantage in the K4 because it gives the opponent the initiative and me the reversal.
As for diving, just don't go full throttle downhill. . .109s need a fair bit of throttle adjustment, and one of the first things you learn is that unless you know it is going to be a very short dive to get E for a quick climb, keep the throttle down. Hell the K4 will compress in level flight at full throttle!