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I had seeded tomatoes capsicum and chillies in a long pot, all sprout up a few leaves come up say around 4 leaves and then they die off. It is very hot and dry in Riyadh and I have kept them outside can you tell me what goes wrong?

It's possible that the soil doesn't drain properly, either it drains too fast or too slowly. The first would mean the plants die from lack of water. The second would mean they die because the roots are drowning. When the weather is hot and dry, people often feel they must provide more water to their plants. That's fine and helps the plants, unless the soil isn't draining properly, and the water collects in it. Poorly draining soil will be dry in the top couple of inches, but soggy deeper in the container.

The container must have sufficient drainage holes in the bottom, or even the best soil will be soggy.

It's possible that the plants are dying from a fungal problem, such as damping off, except that's usually associated with cool, damp conditions, not hot and dry. That's why I believe it to be a problem with the soil and drainage, instead. JMO, though.

Thank you guess more watering could be the problem coz the top soil gets dry very fast,so should I do anything regarding top soil like spraying water with a water spray or so or just leave it
as it looks extremely dry

Does the pot have drainage holes? Does the soil drain rapidly? Dig into it, if you can do so without disturbing any plants, and see if the soil in the bottom half of the pot is soggy. If it is, you need to use different soil. If you can't get better soil, maybe you can improve what you have by amending it.