Q.Old rhubarb plants
We have some very old rhubarb plants that have produced great batches of rhubarb right up until last year, when the stalks have become very short (3-4 inches!) and the leaves seem to have holes in them after I mulched the plants with old straw. I’ve read that the plants should be 3-4 feet apart, but over the years they’ve grown closer together. Do they need to be divided, transplanted elsewhere and should the plants have 3-4 feet to grow?

Overcrowding will significantly reduce yields and even lead to other issues, such as fungal problems from lack of air circulation. Therefore, digging them up and dividing the plants will greatly help, keeping them spaced apart at least 3 feet between one another. This article will help you with dividing:
https://www.gardeningknowhow.com/garden-how-to/propagation/propgen/dividing-plants.htm