This minimum information that Event Espresso needs for a registration is a first name and an email address. Last name is now optional.
Questions for Event Espresso 4 are creatied through the questions editor which can be found in Event Espresso --> Registrations Form --> Questions --> Add New Question.
That editor has an option for making a question optional or required:
By default it is set to optional.
If you set a question to required then it will be required for all events. This is because you would enable this question for a question group and then assign that question group to an event via the event editor.
If you have an event that are different that your other events in that they may require or not require an answer to a question, then you could create a new set of questions for that event.
Be sure to include some event-specific information so it is easy to understand that this question applies to a specific event. Here is an example of an optional question that may be enabled:
Is this your first time attending one of our events?
Now if you would like to make it event-specific, then you could do something like this instead and then set it to required:
Is this your first time attending our Spring Festival event?
References:
https://eventespresso.com/topic/required-fields-for-registration/