4 weeks Peru and Bolivia itinirary

As you are on your own I suppose you will take a tour to the Colca Canyon from Arequipa. Make sure you choose a tour with hiking in the canyon (as opposed to tours that only go to the Colca valley and the Cruz del Condor). You could add a day to the canyon and go to Cabanaconde on your own. All hostels there can help you plan a trek, with or without a guide. If you would like to go on your own like we did you would almost certainly meet people at the hostels to hike with. This website helped us a lot when we were planning our trek:
http://www.pachamamahome.com/tours.htm

You can actually go from Chivay (Colca) to Puno without backtracking to Arequipa. The tourist buses are more expensive than the public bus but it would save time. Look at 4M Express and Rutas del Sur.

I can't help with Bolivia. Yet. We are going in March for 7 weeks! I think that the most popular way from La Paz to Uyuni is by overnight bus. You can also fly.

We spent 9 days in the Cusco/Sacred Valley area and that was without a trek. You can do treks on your own (but not the Inca Trail). For example google "Choquequirao on your own". But if you have no trekking experience that is probably not the way to go. Again, you could find others to go with. Or you can do day hikes. Many great day hikes in that area.

If you do still have time after Cusco I highly recommend northern Peru. Trujillo, Chan Chan and Huaca de la Luna, Chiclayo/Lambayeque, Chachapoyas (Kuelap!, Gocta Falls!...), Leymebamba, Cajamarca... And, for hiking and trekking, Huaraz but I have not been there (yet).

You've allowed enough time for everything, but if you want to add in a trip to Uyuni and do the 3 day tour from there, you might not have enough time to do a trekking tour from Cusco too in one month. But you might have time to do both. If you're concerned about the cost, you'll probably find that booking a trek once you get to Cusco is cheaper than the prices you see on the internet.

How to I integrate Salar de Uyuni in my itinerary?

You can fly from La Paz to Uyuni, or actually you can take an overnight bus from La Paz that should get you to Uyuni early enough to take a jeep tour the same day.

The other thing is, your trip would work much better if you fly into Lima and use a flight from La Paz to leave South America at the end. And visit Cusco before Bolivia. Or if your international airfare is already booked, fly from La Paz to Lima at the end a day or two before flying back home. The land journey to go back and forth from La Paz to Peru would retrace the same route past Lake Titicaca and be time consuming.

Your itinerary looks a bit strange, hence the problem with logistics.
I made a similar trip that took exactly one month. My itinerary was: Lima - Pisco+Paracas - Ica - Nazca - Cuzco+Sacred Valley - Salkantay trek+MP - Cuzco - Puno+ Titicaca Islands - Copacabana - La Paz - Uyuni+Salar - San Pedro de Atacama - Arica - Tacna - Arequipa+Colca Canyon+whitewater rafting - Lima.
All on busses, at normal pace, without rushing.

Thank you for all your advices. 

There are no direct flights from Peru or Bolivia to Germany. The flights to or from Bolivia back were extrem expensive so I already booked a flight to Lima and back from Lima.

I have some offers for the Salkantay Trek with MP between 300 und 400 Dollar. I think this is a good price. No I am thinking about witch treks and how long to stay in Cusco. At the weekend I will take my Guide book and plan again.

@Fieldgate: Thank you very much.

Your itinerary looks a bit strange, hence the problem with logistics.
I made a similar trip that took exactly one month. My itinerary was: Lima - Pisco+Paracas - Ica - Nazca - Cuzco+Sacred Valley - Salkantay trek+MP - Cuzco - Puno+ Titicaca Islands - Copacabana - La Paz - Uyuni+Salar - San Pedro de Atacama - Arica - Tacna - Arequipa+Colca Canyon+whitewater rafting - Lima.
All on busses, at normal pace, without rushing.

I don't want to "steal" this topic, but how much time did you spend at each city? Lima, Cuzco, Titicaca etc. We are looking at either spending 40 days in Peru or do something very close to your itinerary. We're worried we have to rush a bit and not get enough time, but I see you didn't rush doing it in a month.

That itinerary sounds great but seems like a lot in one month. But maybe not. Like I said, I have not been to Bolivia yet. We had one month too and spent the whole month in Peru. 1 day Lima, 11 days northern Peru (highly recommended: Trujillo, Chan Chan and Huaca de la Luna, Chiclayo/Lambayeque, Chachapoyas (Kuelap, Gocta Falls...), Leymebamba, Cajamarca), Lima for one night, 9 days Cusco/Sacred Valley/MP, Arequipa 1 day, Colca Canyon 5 days, Huacachina 1 day, Paracas 2 days, Lima.

With 40 days it looks even better. You'll have more time for Bolivia.
On my first trip I spent only one week in Bolivia which was really short, but a few months later I made another trip, spending 5 weeks in the country.

Another alternative would be spending all 40 days in Peru (south and north), but, the disadvantage is that it's difficult, if at all possible, to make a loop in the country, without backtracking. So, I'd suggest southern Peru + Bolivia for this trip, and to leave northern Peru for another trip.

As for time spent at each place, usually 2-3 days, depending on what's there. For example, Nazca only one night, Cuzco 3+2 nights (before and after the trek).