Ethan Rankin only had his stabilisers removed at the start of the month, and was watching television when he saw Captain Tom Moore walking 100 lengths of his garden to raise £29 million for the NHS.
The nursery pupil, who has a pal being treated for leukaemia, told his mum, Erin, 39, that he wanted to do the same thing on his bike.
So far Ethan, from Alloway, South Ayrshire, has raised £1,365.
He has been going on bike rides with his mum Erin, dad Aaron, and brothers Evan, 12, and Aidan, two.
Erin said: "It was totally his idea.
"He was watching TV the morning Captain Tom did his 100th lap.
"He turned round and said he wanted to cycle 100 miles.
"I said 'no darling, you've only just had your stabilisers off'."
But determined Ethan didn't let up on his goal and his family soon released he was dead set on completing the challenge.
Erin said: "Seeing a wee one having such courage is just phenomenal.
"One of his wee friends isn't well and his papa had surgery just before lockdown.
"At the age of four he's very aware of what the NHS is and that people have to go to hospital for many different reasons.
"At four I didn't have a clue what it was.
"He's decided to do the same as Captain Tom.
"He cycles generally every day, but we're aware because of the lockdown rules we're not to cycle too much.
"When we set up the JustGiving page he was at 28.8 miles.
"We just set up the page late on Friday and he's raised over a thousand already.
"He's oblivious to how much that amount of money is."
So far Ethan has cycled 57 miles, and Erin is confident he'll hit 100 miles by his fifth birthday on May 18.
With help from neighbours, the family is planning on setting up a finish line, complete with balloons and banners, to celebrate his achievement.
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