As deckeresq mentioned, it can vary by discipline. However, if you are interested in pursuing a STEM PhD in the USA, I would not recommend participating in the accelerated Bachelors/Masters program as described.
PhD admissions committees are looking for evidence you can (or have the potential to) perform impactful independent research. The best way to show this is by doing research before you apply. This is why students who have taken project-based Masters programs with published papers tend to have stronger applications. Accelerated Masters programs can be a great option for people who would otherwise go on to take a course-based Masters and enter the workforce, but it's not the best use of your time with your goals.
A better option would be to search for research-based internship positions during the summer and school year. There are generally a great number of external programs/universities/companies that have offer positions in the summer that you can apply for. During the school year, see if you can help in any of the active on-camps labs; ask your professors if they have any suggestions on how to start getting involved.
With a BSc, great grades, undergraduate research, and letters of recommendation that can attest to your research ability, your application will be significantly stronger than with a BSc, course-based MS, great grades, and letters of recommendation that describe your class participation and hard class work, even if you managed to do the latter in four years.