Good quality silicon for aquariums should hold up for 20+ years.
The bubbles are small and appear to be on the glass and not between the 2 sheets of glass. The silicon between the 2 sheets of glass is where it's important. You need to check the seams where the glass joins together and make sure there are no bubbles there. If there are bubbles between the glass, that is a concern.
The air bubbles are normally caused by poor quality silicon. The silicon also appears to be turning cream/ white and this means it is breaking down. Again this is usually poor quality silicon. However, bleach/ chlorine can cause silicon to break down. But you need a lot of chlorine/ bleach and it needs to be in contact with the silicon for a long period of time (more than a couple of hours). The chlorine/ chloramine in tap water is not strong enough to do this. You need straight household bleach or granulated swimming pool chlorine to damage the silicon. If you have never bleached the aquarium, then the problem is caused by cheap quality silicon or a bad batch of silicon, which is a manufacturing defect.
Push the silicon with your finger or thumb. It should be firm but slightly squishy. If the silicon is hard and not squishy, then it has dried out and needs to be replaced.
Depending on where you got the tank and how much it cost, you could contact the manufacturer and see if they will repair or replace it. They probably won't but the silicon that holds aquariums together should last longer than 4 years.
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You can use any brand of silicon made for glass, as long as it says "suitable for aquariums". I use "Selleys", which is in Australia. There should be other brands where you are.
