[en] The Posiva Flow Log, Difference Flow Method (PFL DIFF) uses a flowmeter that incorporates a flow guide and can be used for relatively quick determinations of hydraulic conductivity and hydraulic head in fractures/fractured zones in cored drillholes. This report presents the principles of the method and the results of measurements carried out during the excavation of the underground access tunnel ONKALO at Olkiluoto. Phase 1, phase 2 and phase 3 of the probe hole measurements in the access tunnel started when the tunnel was 15 m long and finished at the tunnel length of 3288 m. The results of phases 1, 2 and 3 are presented in reports 2006-65 (Phase 1) /Reiman, M., Vaeisaesvaara, J and Poellaenen, J. 2006/, 2008-37 (Phase 2) /Pekkanen, J., 2008/ and 2010- 02 /Pekkanen, J and Vaeisaesvaara, J., 2010/. Probe hole measurements were continued normally in phase 4. The phase 4 results are presented in this report. Flow measurements started on April 15, 2009 when tunnel length was 3528 m and ended on November 25, 2009, tunnel length 3976 m. Probe hole measurements will continue normally below 3976 m. In addition to probe holes, shallow core-drilled drillholes were also measured in ONKALO. The drillholes discussed in this report are ONK-PVA3, ONK-PVA6, ONKPP187, ONK-PP190, ONK-PP194, ONK-PP196, ONK-PP223, ONK-PP226 and ONKPP227. The PFL DIFF probe was used to detect flow within single fractures in the drillhole or probe hole. The method utilizes rubber disks to isolate the flow in a test section from that in the rest of the probe hole or to measure the flow along the probe hole. The flow along the probe hole was measured in probe holes from ONK-TR3528 to ONK-TR3976 using 0.1 m and 0.2 m point interval. Core-drilled drillholes were measured using a 0.5 m section length (the distance isolated with the rubber disks) with a 0.1 m point interval. The device used includes a sensor for single point resistance (SPR). SPR was measured in connection with flow measurements. The flow measurement and the single point resistance measurement were used to locate flowing fractures and evaluate their transmissivity. Electrical conductivity (EC) and temperature of water were registered during automatic flow logging. The conductivity values are temperature corrected to 25 deg C. (orig.)