they don't give out the dimensions of the sub
Pisces IV and VI are basically the same design, the only difference between the entire Pisces-series, is some updated equipment (machinery, electronics, etc.), the hull, sail everything is the same. The biggest changes, between them, is some of the outer plating that is protecting whats on the outside of the pressure hull, differ a bit, due to the equipment need more space. For example Pisces IV sail, is the same as Pisces VI sail, the only difference there is that Pisces VI have an additional structure behind the sail, for some equipment.
The pressure hull trough the entire class is the same, I would not be surprised, if two of them is the same submarine, but given a new name/designation due to changes done