ChatGPT Plus: default GPT4 web browsing. Userscript for Tampermonkey or Greasemonkey that sets the LLM default model in the https://chat.openai.com/ User Interface.

defaultGPT4-web-browsing.js

const clickElement = (selector) => {
  const element = document.querySelector(selector);
  if (element) {
    element.click();
  } else {
    setTimeout(() => clickElement(selector), 100);
  }
};

clickElement("[id*='trigger-gpt-4']");
clickElement("[id*='trigger-gpt-4-browsing']");

defaultGPT4-web-browsing.user.js

// ==UserScript==
// @name         Chat OpenAI Auto Click
// @match        https://chat.openai.com/
// @match        https://chat.openai.com/?model=text-davinci-002-render-sha
// ==/UserScript==

(function() {
  const clickElement = (selector) => {
    const element = document.querySelector(selector);
    if (element) {
      element.click();
    } else {
      setTimeout(() => clickElement(selector), 100);
    }
  };

  clickElement("[id*='trigger-gpt-4']");
  clickElement("[id*='trigger-gpt-4-browsing']");
})();

NEW0517-defaultGPT4-web-browsing.user.js

// ==UserScript==
// @name         GPT-4 Button Auto Click
// @match        https://chat.openai.com/
// @match        https://chat.openai.com/?model=text-davinci-002-render-sha
// ==/UserScript==

const clickFirstDivInsideButton = () => {
    let buttonNode = document.evaluate("//button[contains(., 'GPT-4')]", document, null, XPathResult.FIRST_ORDERED_NODE_TYPE, null).singleNodeValue;
    if (buttonNode) {
        let firstDivInsideButton = buttonNode.querySelector('div');
        firstDivInsideButton ? firstDivInsideButton.click() : setTimeout(() => clickFirstDivInsideButton(), 100);
    } else {
        setTimeout(() => clickFirstDivInsideButton(), 100);
    }
};

clickFirstDivInsideButton();

Relevant chats:
https://chat.openai.com/c/4c526c4a-fe2e-450a-b143-31a2922a1760
https://chat.openai.com/c/ca3f054d-bc88-4d92-9e1d-94214f84d57d

Enforces ChatGPT4 | does not even allow for the user to change to GPT3.5

// ==UserScript==
// @name         Chat OpenAI Auto Click
// @match        https://chat.openai.com/
// @match        https://chat.openai.com/?model=text-davinci-002-render-sha
// ==/UserScript==
const clickFirstDivInsideButton = () => {
  const clickHandler = () => {
    const buttonNode = document.evaluate(
      "//button[contains(., 'GPT-4')]",
      document,
      null,
      XPathResult.FIRST_ORDERED_NODE_TYPE,
      null
    ).singleNodeValue;

    if (buttonNode) {
      const firstDivInsideButton = buttonNode.querySelector('div');
      if (firstDivInsideButton) {
        firstDivInsideButton.click();
      }
    }
  };

  const observer = new MutationObserver((mutationsList) => {
    for (const mutation of mutationsList) {
      if (mutation.type === 'childList') {
        clickHandler();
        break; // Break the loop after the first mutation is detected
      }
    }
  });

  observer.observe(document.documentElement, { childList: true, subtree: true });
};

clickFirstDivInsideButton();