Evaluate posthog/posthog-php v4 for breaking changes

posthog/posthog-php v4.0 has been released. We need to review the changelog and migration guide for any breaking changes that affect our PostHog driver before adding `^4.0` to our composer.json constraint.

- [ ] Review v4 changelog / upgrade guide
- [ ] Check for breaking changes in our PostHog driver usage
- [ ] Update driver code if needed
- [ ] Add `^4.0` to composer.json