Hi,

To do that in your manifest in web._assets_bootstrap 
Add these lines

'web._assets_bootstrap': [
            (‘after’,’web/static/src/scss/import_bootstrap.scss', your_module/static/src/scss/import_bootstrap_4.scss’),
]

Make sure that you have a file named import_bootstrap_4 which has bootstrap v4 in it

Alternatively you could also try
'web._assets_bootstrap': [
            (‘remove’,’web/static/src/scss/import_bootstrap.scss')
            your_module/static/src/scss/import_bootstrap_4.scss’,
]

Hope it helps