UML activity diagram: Action and Activity

Activity diagrams don't use the concept of objects, as they usually don't show who does what, but only which actions/decisions are taken. You could use partitions (also called swim lanes) to indicate who executes the actions, but if you have more than two or three objects interacting, this can become rather unreadable.

For describing the interactions between several objects, it will be better to use a sequence diagram or a communication diagram (sometimes referred to by its old name of collaboration diagram).

Although I generally agree with Bart van Ingen Schenau's answer, I think a few points need additional elaboration.

Th advantage of the 4+1 View Model is that it maps stakeholders to the type of information that they need, without requiring specific modeling notations to be used. The emphasis is on ensuring that all groups have the information to understand the system and continue to do their job.

The 4+1 View Model of Software Architecture was described in Philippe Kruchten's paper Architectural Blueprints - The "4+1" View Model of Software Architeture that was originally published in IEEE Software (November 1995). This publication doesn't make specific references to UML. In fact, the paper uses the Booch notation for the logical view, extensions to the Booch notation for process view and development view, calls out the use of "several forms" of developing a physical view, and a new notation for scenarios.

Instead of trying to map each of the views to particular types of diagrams, consider who the target audience of each view is and what information they need. Knowing that, look at various types of models and which one(s) provide the required information.

The logical view is designed to address the end user's concerns about ensuring that all of their desired functionality is captured by the system. In an object-oriented system, this is often at the class level. In complex systems, you may need a package view and decompose the packages into multiple class diagrams. In other paradigms, you may be interested in representing modules and the functions they provide. The end result should be a mapping of the required functionality to components that provide that functionality.

The process view is designed for people designing the whole system and then integrating the subsystems or the system into a system of systems. This view shows tasks and processes that the system has, interfaces to the outside world and/or between components within the system, the messages sent and received, and how performance, availability, fault-tolerance, and integrity are being addressed.

The development view is primarily for developers who will be building the modules and the subsystems. It should show dependencies and relationships between modules, how modules are organized, reuse, and portability.

The physical view is primarily for system designers and administrators who need to understand the physical locations of the software, physical connections between nodes, deployment and installation, and scalability.

Finally, the scenarios help to capture the requirements so that all the stakeholders understand how the system is intended to be used.

Once you understand what each view is supposed to provide, you can choose what modeling notations to use and at what level of detail is required. Bart's last paragraph is especially true - you can show various levels of details in your UML models by focusing on particular design elements or combining various types of diagrams into a set. In addition, you may want to consider going beyond UML to other modeling notations to better describe your system architecture - SysML, Entity-Relation modeling, or IDEF.