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Hi Stefano, I've done that in the past on my wooden neck basses - had zero issues. It would take a damn sight more more to cause a graphite neck to structurally fail! so be assured the additional tension will not effect it. - at most you may have to do a very minor truss rod adjustment, but considering the original graphite necks didn't even have a truss rod... i'd doubt you will even have to do that! Few thing to consider: The size of the nut (you may need to file it down for larger gauge B strings) How "floppy" the strings are (can lead to fretbuzz etc) - this can be fixed by using higher gauge strings. or a longer scale length of the guitar, 4 string bass guitars tend to be 34 Inches - i know of a few players who custom ordered basses to 35 inches, so the strings are a higher tension thus play lighter gauge strings in the B-E-A-D configuration without having to worry about buzz. Pretty much all 5 & 6 string basses tend to be 35inch + scale. Slightly off topic but i believe Carl Thompsons 4 string basses are 36inch scale! Hope that helps, Jonathan