This fourth volume of 'A Certain Scientific Railgun' sees the beginning of the 'Sisters' story arc and the exploration of the dark side of Academy City. The volume begins with Mikoto and Kuroko discovering a cash card in one of Academy City's back allies and finding out that a number of similar cards have also been found. While investigating who could be responsible for this, Mikoto makes a few unwelcome discoveries about some of Academy City's secret projects and her own unwitting part in them.
This is a great volume of 'A Certain Scientific Railgun' and although there is little action, the development of the plot is well done and quite interesting. The volume focuses mostly on Mikoto but it does introduce a number of other characters with the 'Sisters' themselves being quite sweet. The volume has some nice humour throughout and this is nicely juxtaposed with the more emotional moments towards the end of the volume.
The 'Sisters' arc is one my favourites of the series and the first to real crossover with 'A Certain Magical Index' (although that isn't until further on in the arc and you don't have to read the other series to understand the story). This volume is a very nice one and is well worth a full five stars.