Appendix C. Choosing an LLM
C.1 Popular Large Language Models
This appendix outlines the key features of the most popular large language models available at the time of writing. It also highlights the criteria to consider when selecting the most suitable LLM for your project.
C.1.1 OpenAI GPT-4o and GPT-4.1 series
OpenAI’s GPT-4, launched in March 2023, marked a significant milestone as the first “frontier model” to demonstrate advanced reasoning capabilities. It was also one of the earliest multi-modal models, capable of processing text, images, and later video, enabling diverse applications. While its architecture details remain undisclosed, GPT-4 was rumored to feature 1.8 trillion parameters and employed a “Mixture of Experts” design to enhance accuracy across various use cases.
The GPT-4o series succeeded GPT-4, offering specialized options for different tasks. GPT-4o was optimized for complex, high-accuracy tasks like multi-step reasoning, while GPT-4o-mini targeted simpler applications such as summarization, editing, and translation, replacing GPT-3.5 for these use cases.
The GPT-4.1 series, released in mid-2025, followed GPT-4o and introduced refinements in performance, speed, and cost-efficiency. It retained GPT-4o’s strength in complex reasoning while offering better responsiveness and adaptability across a wide range of tasks. With improved balance between accuracy and resource usage, GPT-4.1 became a strong fit for both advanced and general-purpose applications.