best book for emergency elective in 4th year?

Depends...do you want to ultimately go into EM or is this just a month? If you're not going into EM, something like EM Secrets or First Aid for your Emergency medicine rotation will do fine enough.

If you're actually going into EM, you may want a more in depth book. Lange has a decent quick reference/just the facts book, while if youre 100% set on EM consider getting Tintinalli or Rosen, or even Harwood Nuss in addition to a Lange like book for future reading/in depth review.

Overall, Secrets or Lange would be my non EM/EM in the furture books.

There's a new primer out for med students (see below). It's free and put out by CDEM.

I like this book. It has general chapters that describe how to work up complaints like "headache," as well as more specific chapters on causes of headaches like SAH. Some content is doubled, making it heavier, but it's a good format for beginners. When I was on my Peds EM elective, it helped me look like a rockstar w/r/t EBM, workups, dispo.

Ask whether the rotation includes a loaner book. Seems to be a common policy of EM rotations.

I found Lange USMLE Roadmap: emergency medicine very helpful for the rotation. It is fairly cheap (approx $30) and presents different organ systems in a systematic approach that will help you both during your rotation to work up different differentials and management. I highly recommend this for someone not going into EM and for those going into it as a supplement.