“If you don't” refers to a state such as in which, when resolving this Digimon’s [When Attacking] effect, your opponent doesn't have a Digimon with 4000 DP or less in play, or your opponent has a Digimon that can't be deleted with the effect. Essentially, if your opponent doesn't have a Digimon that can be deleted by “Delete 1 of your opponent’s Digimon with 4000 DP or less,” follow the instructions listed after “If you don't”.

No. If you activate this card’s [When Attacking] effect and your opponent has a Digimon with 4000 DP or less in play, you must choose that Digimon as the target of the effect. If a valid target exists, you must choose it.

Yes. If you chose your opponent’s Digimon with 4000 DP or less as a target, it counts for this card’s effect, even if the chosen Digimon has an effect that prevents it from being deleted.