There are brushes you should use with extensions (loop brushes); these allow you brush the hair and minimize tangles.
Extensions do not necessarily need to be replaced every 3-4 months, but they need to be removed and reapplied because the further the bond gets from your scalp, the more stress it places on the hair shaft. The extra weight as it gets further from the scalp puts the hair (from scalp to bond) under more stress (and pulling) causing a higher chance of damage; think about pulling on your hair all day long, eventually it will break. It's very important to have reapplication done, but if the extension hair is high-quality and well-maintained (and depending on the application method chosen), it is possible utilize the same extensions for multiple applications which can significantly reduce the overall cost of long-term extensions.
For the same reason as outlined above, if you have someone who doesn't place them properly, you run the risk of unnecessary damage and additional expense (by not being able to go as long between reapplications.) If you need them reapplied (because of placement) every 2 months for example, then either 1. your extension specialist is not placing them properly to begin with(i.e. placing them too low and is not very good) or 2. your hair grows mad fast.