Nick Gauntlett was heading to the Land Rover Blair Castle International Horse Trials at Pitlochry, Perthshire when the accident happened on Tuesday night.
After the truck broke down on the A9 – just 30 minutes from Blair Castle – all team members left the vehicle while the horses remained inside.
While waiting for recovery services, Mr Gauntlett and his team watched as a lorry ploughed into the side of their truck.
His horse, Party Trick, died alongside three animals belonging to US-born eventer Sophie Hulme.
Police Scotland are investigating the circumstances of the crash and said two people were taken to hospital for treatment.
Mr Gauntlett, who is an international eventer and coach, is based in Chipping Sodbury, South Gloucestershire.
Writing on Facebook, he said: “My amazing my family are fine and that is definitely the main thing.
“We lost Party Trick Having broken down just 30 mins from Land Rover Blair Castle International Horse Trials, we sat on the grass verge and watched a lorry plough into our truck.
“You don’t need the details, but we lost all four horses on board. We are obviously all heartbroken.”
Gloucestershire-based Ms Hulme, who was already at Blair Castle, said: “I am so absolutely devastated and at a loss for words.
“Three of my lovely horses my precious babies are gone. Taken far too soon from this world.
“My beautiful Charisma and Feyre are gone and my lovely Fintan as well. I’m so sorry for Nick and Amanda and for their loss. The world can be such a cruel place.
“My world is completely upside down at the moment.
“Thank you to everyone who has messaged and reached out to me I am slowly working my way through all the messages.
“It’s going to be hard for a while and it’s going to be tough but the best we can do is soldier on.
“Everyone please give your horses an extra hug and kiss from me as you never know what might happen.”
In a statement, the Land Rover Blair Castle International Horse Trials said: “Both British Eventing and the organisers of Land Rover Blair Castle International Horse Trials send their heartfelt sympathies to Nick, his wife and children, Sophie Hulme, Nick’s team, the owners and the horses’ grooms, and all involved with them.
“Nick lost Party Trick, while Sophie Hulme lost three horses. Heart-breaking.
“Eventing is a family and what support we can provide we will. We are grateful that no-one in Nick’s lorry was injured in the accident.”
A Police Scotland spokesman said: “Around 2.45pm on August 24, officers were called to a report of a road crash involving two lorries on the A9 at Cairnie Braes, near Perth.
“Police, fire and ambulance attended and two men from one of the lorries were taken to Ninewells Hospital in Dundee.
“One of the lorries was carrying four horses. We can confirm that two of the horses died and two had to be euthanised by vets.
“The road was closed until around 9.30pm. Inquiries into the circumstances surrounding the crash remain ongoing.”