About 30% of the sources in the 4th INTEGRAL-IBIS catalogue are unidentified in that they lack an optical counterpart. To be able to classify them, X-ray observations are of crucial importance as they can place tighter constraints on the high energy error box, which is usually of the order of a few arcminutes, and allow their broad band spectrum to be studied. To this aim we have cross-correlated the list of all unidentified IBIS sources in the fourth catalogue with the archive of all XMM-Newton pointings, finding a set of 6 objects with archival data. For 1 of them, IGR J17331-2406, no X-ray source is detected by XMM inside the IBIS error box, most likely due to the fact that it is a transient object. In the case of IGR J17445-2747 two possible X-ray counterparts are found inside the IBIS error box: one is very weak while the other is bright but only detected once. In each of the remaining 4 cases: IGR J155359-5750, AX J1739.3-2923, AX J1740.2-2903 and IGR J18538-0102, we find instead a convincing association for which we provide an improved X-ray position and information on the optical/infrared counterpart. We also performed a detailed analysis of their XMM-IBIS spectra and on the basis of all information acquired we suggest that IGR J155359-5750 is an AGN of intermediate type, AX J1739.3-2923 and AX J1740.2-2903 are High Mass X-ray Binary systems, IGR J17331-2406 and IGR J17445-2747 are Galactic transient sources and IGR J18538-0102 could be a background AGN. ; Comment: 9 pages, 10 figures, accepted for publication on MNRAS main journal