The Department for Transport said it will buy deciduous and evergreen trees of up to 12ft high from home suppliers and plant them to help offset the carbon impact, visual scarring of the land and noise cause by the line.
A spokesman said the choice of suppliers is being "ascertained" and names will be announced before construction starts in 2017. Costs have not been finalised.
He added: "Ministers have asked us to look into sourcing trees from British nurseries," he added, saying officials would look at EU tender rules between now and 2017.
"Ministers are keen to ensure new planting is carefully planned and uses native species sourced from British tree nurseries to create new habitats for wildlife and flora that match the character of the local area and offset some carbon impacts.
"Government officials will look at options for fast-tracking planting to the start of the construction stage to give them time to grow and maximise screening benefits."
But Chilterns Conservation Board chief officer Steve Rodrick likened the move to using a "sticking plaster to cover a gaping wound". He added that four million trees sounds impressive but after natural loss and thinning it will amount to around 100 per hectare.
"When you say four million trees you expect it to be based on a well-thought-through landscape plan and planting programme, but nothing has been published," said Rodrick.