Sure, it sounds like a fantastic journey you're planning across some of France's most charming towns. Here are some travel tips to make your trip smooth and enjoyable.

Best Time to Visit:
The best time to visit these coastal towns in France is between April and October. This is when the weather is usually the most pleasant, with average temperatures ranging from 15°C to 25°C. September is especially beautiful in Brittany because it's less crowded but still has lovely warm weather. Keep in mind that these places can get quite busy during summer (July-August) due to the holiday season.

Route and Accessibility:
The most convenient place to start your trip might be Saint-Nazaire, which is well connected by train to other major cities, including Paris. Then Quiberon, Brest, and finally Roscoff could be visited in that order. Train and bus services are available between these towns, and renting a car might be another good option, especially to explore local surroundings.

Accommodation and Booking:
There are a variety of accommodations available in all the towns, ranging from luxury hotels to budget-friendly options like B&Bs and campsites. It's always a good idea to book in advance, especially during the peak season. Websites like Booking.com, Airbnb, and the local tourist office websites may offer a range of choices.

Clothing and Essentials:
As these towns are in a coastal region, the weather can change quickly. Bring a mix of both warm and light clothing, including a waterproof layer. Essentials should include a good pair of walking shoes, sunblock, sunglasses, and a reusable water bottle. If you plan to visit the beach, don't forget your swimsuit!

Health and Safety:
Standard travel insurance that covers medical expenses is recommended. France has an excellent healthcare service. Don't forget to keep emergency numbers handy. It’s generally safe to travel throughout France but be mindful of your belongings in the touristy areas.

Local Etiquette and Respect:
French people appreciate their language, so learning a few basic French phrases can be handy. It's also common etiquette to say 'Bonjour' (Hello) when you enter a shop and 'Au revoir' (Goodbye) when you leave. These towns are located in Brittany, which has its own unique culture within France, be respectful of local traditions and customs.