The two memorable milestones for an only child are ones parents Gavin and Stacy would have treasured for the rest of their lives.
But Dylan never even got to try on his school uniform or plan a party because he died of a rare muscle cancer just months before the exciting day - Monday, August 18.
His death has left a massive void in his parents' lives but their son's courage and strength has inspired them to throw a birthday party in memory of their "superhero".
The couple, whose home remains a shrine to their son with a Lego superhero village in their living room, hope to raise money for Yorkhill Children's Hospital in Glasgow, where staff battled to save his life.
Gavin, from Kilmarnock, said: "It is absolutely heartbreaking to think we'll never see Dylan's first day at school or any more birthdays.
"He was supposed to start school on the same day as his firth birthday and he was so excited about it. It should have been the most exciting day of his life for him and for us but he didn't make it.
"His birthday and first day at school were all he spoke about. He was determined he wanted to wear a school tie and pick out his own school bag.
"We have great memories of our wonderful son but it's not enough.
"It's going to be a difficult day for us and sad because he should be here with his mum and dad celebrating his birthday and his first day at school.
"We needed more time with our son. It's not fair that he should go before us. We had so much to look forward to and Dylan was our world.
"I haven't just lost my son, I've lost my best friend as well and it's breaking our hearts.
"We think about him every moment of every day and always wonder what he would be doing now. It is still raw, we only lost him six months ago. It feels like yesterday.
"We are crumbling here without Dylan and rattling around in this house. Dylan was our superhero, our inspiration and it was his strength and courage that kept us going when we were told he had nine months to a year to live."
The couple said during the four precious years they had with Dylan, he touched so many lives.
Gavin added: "Everyone who met him absolutely adored him. You would never have known to look at Dylan that he was dying inside.
"For a four-year-old kid he was the most amazing human being I've ever met. It was an honour to be his dad but we just didn't get enough time together."
Dylan passed away in his parents' arms on January 17 this year and 800 mourners paid their respects at his funeral at Masonhill Crematorium in Ayr.
He was diagnosed with alveolar rhabdomyosarcoma, a rare childhood muscle cancer, when he was just two after his mum spotted a lump in his leg.
Within weeks he was having intense chemotherapy, surgery and radiotherapy which almost killed him.
But Dylan's fighting spirit got him through the gruelling nine-month treatment and he went into remission in August 2012.
He had to learn to walk again after surgery on his leg and was still receiving treatment when he asked his parents to sign him up for skiing lessons at Xscape at Braehead.
BUT the cancer came back six months later and again it was Stacy, a carer, who noticed another lump, in Dylan's groin.
Doctors operated but the second tumour was too dangerously close to the main artery in his leg to remove it completely and he was given up to a year to live.
Gavin, an operations and development manager, decided to take a year out of work to treasure every last moment with his son.
Dylan's tragic story touched the hearts of the local community and they raised thousands of pounds to send Dylan and his parents on a trip of a lifetime to Disney World in Florida last May.
His parents also took him to every theme park and children's activity they could think of, including Legoland and Cadbury World.
Gavin and Stacy will mark their son's fifth birthday with a big party at St Joseph's Academy, Kilmarnock, tomorrow, from 1pm until 4pm, as a thank you the local community for all their support.
They hope hundreds of adults and children will turn up to Dylan's big day to raise thousands of pounds for the Schiehallion ward at Yorkhill, where staff battled to save him.
Activities will include face painting, bouncy castles, Owl Magic, Kirsty's Kritters, loom bands and nail art.
Gavin added: "We are having a birthday party in memory of our son and to give something back to the community and the hospital where Dylan spent a lot of time.
"Everyone has been so supportive and we can't thank them enough for all they have done for us."