A good colourist will want to use a colour remover first to remove the artificial colour before using bleach to lift hair that’s been coloured to a level 4.
It might initially seem cheaper to go down the bleach highlights route but it will take several sessions, not 2, and will cause some degree of damage to the hair. Plus the costs will add up over several sessions.
Dark natural coloured roots against pale blonde hair will show within 4 weeks so you should also factor in the ongoing regular expense involved in the upkeep.