A 4 year old ball could also be fasting because of the on coming breeding season. Although I would recommmend that you get your hot side temps to at least 90-92 degrees for digestion reasons.
4 months is not a very long time, I've personally had snakes fast for up to 8 months. You should invest in a digital scale (one used for cooking) and take a weight every 2 to 4 weeks. If you start to see significant weight loss then you should worry but until then I wouldn't try to assist feed as it can cause more harm than good.
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