The AQ-4035 is an updated replacement for the AQ-4025 The AQ-4035 cartridge set fits all Aquasana countertops and all Aquasana under counter systems bought before April of 2013. The A and B replacement filters are guaranteed to be compatible with AQ-4000, AQ-4500, AQ-4501, AQ-4000W, AQ-4000P, AQ-4000B, AQ-4000N, AQ-4600, AQ-4601, AQ-4601.55, AQ-4601.56, AQ-4601.62 and AQ-4601. The AQ-4035 is our latest generation of replacement filters. We"ve spent the past 18 months redesigning and reformulating our filters to achieve our best filtration yet resulting in the new Aquasana AQ-4035 technology. The Aquasana AQ-4035 design and functionality are so unique, we"ve applied for a new patent. The drinking water filter set is the heart of our Aquasana drinking water systems. It features three distinct filter technologies: activated carbon, ion exchange and sub-micron filtration. Due to the unique manufacturing process, the Aquasana AQ-4035 filter is so densely packed with a sub-micron structure that water has to pass over 2.9 million square feet of contaminant grabbing media. That is how we achieve such high contaminant reduction rates. At the same time, the Aquasana filtration technology is selective, allowing naturally occurring healthy minerals to pass through minerals like calcium, potassium and magnesium. And that is why your water tastes so great.