I am a violinist What piece do you recomend me study this summer?

Alberich, you've got to be kidding? Sibelius for someone who knows up to fourth position? The Sibelius Violin Concerto is a great piece but extremely difficult. I cannot recommend more strongly that you pick something much less difficult. If you are looking for a concerto, the Bach Concerto No. 2 in E Major might be a nice choice, as would the Accolay Concerto or the Haydn G Major. Severn's Polish Dance is a nice short piece you might also like to work on. If you have a private teacher, ask what he/she suggests. If you don't have a private teacher, it would be a great idea to try to get some lessons this summer. Don't get them at a music store, though, because they overcharge. Maybe your conductor could suggest a violin teacher. You might want to try to go to a summer music camp, too (or instead if you're on a budget). There is probably a local one near you that will keep you practicing and interested over the summer. Good luck!