I have several different varieties of very small succulents (mainly Lithops and Conophytum) in small pots indoors. They have been in the same pots and soil for about 4 years.
I'm not sure when it's the right time to move them to a bigger pot, because they don't seem to grow in size.
Some Conophytum even started to die back and I wonder if it's a lack of soil fertility since I have never changed their soil or fertilized them.
Yes, photos would help. Neither type of plants will grow too big. If in enriched soil, they will usually just propagate laterally. If they haven't done that, it might be a sign that they could use some feeding. They can definitely use feeding sparingly, about once or twice a year. They probably don't need much more than that. I'd feed them sparingly with some organic fertilizer.
If they have been in the same pot for 4 years, either replace with new, fertilized soil in the same pots or, put them in slightly bigger pots.
Conophytum have a resting period in summer, when the existing leaves dry up and form a sheath over the newer leaves - during this time, watering should be much reduced. This process is likely what's taken place, rather than your plant actually starting to die, so don't worry about that. From late summer onwards (end of July), resume normal watering, watering well. If you think it needs repotting, use a shallow pot or pan because they're very shallow rooting and don't need a lot of soil depth. You can repot every two years, but they're usually fine if left a lot longer, they're pretty slow growing, but should be given a low nitrogen liquid fertilizer a couple of times a year if you don't repot into fresh potting medium.
Lithops are usually not watered during winter and watering is started again around May, but if you live somewhere with warm winters, a small amount of water might be necessary during winter occasionally. The advice for potting with Lithops is roughly every couple of years, removing all traces of the old potting compost - this is carried out just before the first watering after winter.
If your plants really haven't grown at all in four years, it may be you're not giving them the right treatment, so its worth reading the information on both plants in the link below - there is an extensive amount of background information on Lithops because there are so many varieties, but the parts referring to growth cycle, watering and potting are towards the end http://www.smale-conophytum.co.uk/grow_conos.htm
