Sure! Here is some advice for your upcoming trip to Seoul, Busan, Jeju and Daegu in South Korea.

Best Time to Visit:
The best time to visit South Korea is during spring (April to June) and fall (September to November), when temperatures are mild and the countryside is at its most picturesque.

Route and Accessibility:
All four cities are well connected by South Korea's excellent public transportation system. You can take the KTX train from Seoul to Busan (about 3 hours), and from Busan you can fly or take a ferry to Jeju Island. Daegu is a short train ride from Busan or Seoul.

Altitude and Acclimatization:
Altitude sickness is not generally a concern in these cities as they are all at relatively low elevations.

Accommodation and Booking:
It is suggested to book your accommodations ahead of time, especially if you are traveling during peak tourist season. All these cities have a range of accommodations to choose from, including hostels, guesthouses and luxury hotels.

Clothing and Essentials:
Pack for the weather depending on when you're going. It can get hot in the summer and very cold in the winter. Comfortable walking shoes are a must, as you'll be doing a lot of walking and exploring.

Health and Safety:
Ensure you have travel insurance for any health issues. Korean food can be spicy, so those with sensitive stomachs should be cautious.

Local Etiquette and Respect:
Koreans value respect and courtesy. It's customary to bow when greeting someone. Many Koreans are not fluent in English, but they will appreciate any attempts to speak their language.

Enjoy your trip! Korea has many amazing experiences to offer.