B757 Captain reports being confused after being issued taxi instructions to taxi via B and M to hold short of Runway 4L and expedite through the 4L approach. A right turn is initiated at M resulting in a possible runway incursion. Poor signage is cited as a factor.

Narrative

Received taxi clearance; 'Runway 9; Taxi via B; E; hold short 4L.' Switched to Tower frequency while taxiing on B approaching E. First Officer advised Tower that we were waiting on weights. Taxi clearance changed to; 'B; M; cleared to cross 4L approach; will bring you in the other side.' Received weights; but they disagreed with reconciliation card from the Gate Agent. First Officer sent message of passenger count from card. While waiting on response; and as I was taxiing; I was instructed by Tower to expedite through the 4L approach as traffic was on 3 mile final. Rounding the bend on B; we had the traffic in sight. The east side of B had little room to clear the 4L approach area and signage was poor. I saw the M sign for the right turn; but missed the M sign for the left turn toward Runway 9 as it was greater than 90 degrees behind to the left and I was splitting my attention with the traffic on final. I started a right turn on M instead of a left turn and was instructed by Tower to hold short of Runway 14; but there are no pavement hold short lines for the runway or on the taxi chart. Tower then told us to continue taxi on Runway 14 to hold short of Runway 4R for a 4R departure. The runway signage was not visible until the turn was started on M towards Runway 14.There was a possible runway encroachment to a runway not in use. Contributing factors: 1. Reconciliation of weights with new procedure. 2. Change in taxi instructions with busy Tower frequency due to late weights. 3. Complex taxi instructions with expedite due to traffic (Tower was busy; but would have been better with clearance to say B; left on M to Runway 9).4. Signage at the taxi B and M intersection location with no pavement markings for Runway 14.