Likely the soil is becoming exhausted of ready nutrients. Perhaps think back to the type of soil they are grown in and compare to the required growing conditions; from the wiki page the tamarind tolerates a wide range of soils, prefers acidic conditions, and as a legume can generate its own nitrogen, so you can strike that one off the list. If you are able and confident, knock one of the plants out of its pot and examine how much the roots have explored the available soil volume. Tamarind tolerates pots quite well, but may need feeding with a low N compound once soil has been displaced by root volume.
Once the yellowing is clear, compare the patterns with one of the many sites that explore mineral deficiencies. You will find that patterns emerge, such as following veins, leaf edges, spotting and so on. This may give you a clue what is wrong with the soil.
For an immediate test, take one or two of the four, take plant out of pot, gently knock or wash off the soil, then repot carefully in fresh soil and watch them do better.