I am currently using a Wizard and I just got my hands on a +4 dagger. More specifically the Rabbit Slayer from Dragonlance (ed.: a 2e item).
Question 1: According to official rules can it be used as a spellcasting focus
Question 2: If so how do I go about casting with a +4 focus in terms of modifiers
No magic items above +3 exist in 5th edition
By design magic weapons and items can only add up to +3 at maximum within the system's bounded accuracy. I've searched through the whole of the DMG treasure section and even legendary weapons only have a +3.
Wizards can use Arcane Focus items as an Arcane Focus
They are defined as:
As such I would not deem daggers in general, nor weapon enchanted daggers as Arcane Foci.