Which creatures help Tirian in Chap. 4?  How do they help him?    

There were three Mice, a Rabbit and two Moles who came back to help Tirian. They were carrying little bags on their backs with some food and drink which was the King’s supper. After the dinner, they also helped wash the King’s face.  

What does Tirian witness in front of the fire?  Are there any clues which should indicate that the creature he sees is not Aslan?      

He saw a great crowd of Beasts and Men around the fire and the Ape said something to the crowd and it went and bowed three times to the ground in front of the door of stable. Then Ape opened the door and something on four legs, something that walked stiffly with yellow and hairy, came out of the stable and stood facing the crowd

First, he had never seen the Great Lion, he had never seen a common lion. So he couldn’t be sure that what he saw was a real Aslan.  Second, the creature stood stiffly and said nothing, which didn’t like as he had expected. At last, he remembered the nonsense about Tash and Aslan being the same, which made him to realize that the whole thing might be a cheat. To these thoughts and uncertainty, it indicated that the creature he saw was not Aslan.

Not only has he never seen Aslan before, but Tirian has never seen any other lion either.  What thought reassures him that the stiff, clumsy, yellow thing could not be Aslan?  What happens when Tirian calls to Aslan for help

Like the question 2, one thought was that he might guess the whole thing was a cheat based on the nonsense Tash and Aslan being same. The second one was that from Narnia old times, children always appeared in Narnia as well as Aslan came out when things were at their worst. From all these evidences, he thought the yellow, stiff, clumsy thing was not Aslan.

When he asked Aslan for help, there is nothing change around him but a little change inside him. Tirian began to feel a faint hope and he felt somehow stronger. After crying out in a great voice, he was suddenly plunged into a dream more vivid than any he had had in his life. That dream made him appeared in a lighted room where seven friends of Narnia sat round a table.