Do I really only need 4 shirts for a month?

Yes. European cities have laundromats on almost every corner, and most Airbnbs or hostels offer laundry facilities. Doing a small load once a week keeps your pack light.

Can I bring a suitcase instead of a backpack?

You can, but avoid it if you plan to use trains or buses frequently. European city centers often have cobblestone streets and stairs, making wheeled luggage a nightmare.

So pack je deinen Rucksack für einen Monat Europa

Limit yourself to a 40-liter backpack to avoid checked bag fees and mobility issues on cobblestones. Stick to a 'capsule' wardrobe of neutral colors that can be layered, and commit to doing laundry once a week.

Choose the right bag. Use a 35L to 40L carry-on compatible backpack. Anything larger will be too heavy for train travel and difficult to store in European hostel lockers.
Use packing cubes. Separate your clothes into compression packing cubes: one for tops, one for bottoms, and one for undergarments/socks. This keeps your bag organized and prevents you from dumping everything out to find one shirt.
Follow the 5-4-3-2-1 rule. For a month, pack 5 pairs of socks/underwear, 4 tops, 3 bottoms, 2 pairs of shoes (one on your feet, one in the bag), and 1 jacket. Anything more than this is unnecessary weight.
Optimize for weight. Your total pack weight should not exceed 8-10kg. If you can't lift it over your head with one arm, you have packed too much.

Do I really only need 4 shirts for a month?: Yes. European cities have laundromats on almost every corner, and most Airbnbs or hostels offer laundry facilities. Doing a small load once a week keeps your pack light.
Can I bring a suitcase instead of a backpack?: You can, but avoid it if you plan to use trains or buses frequently. European city centers often have cobblestone streets and stairs, making wheeled luggage a nightmare.