Security impact of access modifiers (public, private, internal, protected)

No, access modifiers don't offer security protection. They are merely there for developer convenience, e.g. they help to enforce good coding practices and help with programming patterns.

It's easy to access otherwise inaccessible modifiers by using reflection in Java/C# and other languages.

The main purpose of the access modifiers is to enforce a specific design, not any kind of security.