Can I keep them in for more than 4 months?

No. You must have maintenance done on your hair extensions by the 4th month. As your hair sheds on a daily basis as part its natural process it is normal to loose up to 50 hairs a day. This combined with natural hair growth of around ½ inch per month can cause matting within the root area of the extension, as well as added weight pulling on the lengthened root area. If you were to keep them in for longer than 4 months you risk damage to your own hair.