The biggest question would be your career planning.
Do you want to be a physician-scientist in clinical research vs physician-scientist in lab research vs a clinician educator vs private practice?
If you want to be a physician-scientist in lab research, you will ideally need extensive prior lab research experience (ideally a PhD). An one-year research fellowship + three-year clinical fellowship may be difficult but not impossible, depending on your mentor support and project.
If you want to be a physician-scientist in clinical research, one-year research fellowship + three-year clinical fellowship make more sense but mentor support would be very important regarding your career. By saying physician-scientist, you need to spend 80% time doing research and applying for grant to support your salary and research. This is a highly competitive field, with very few standing out overall. And you will make significantly less money at least for many years or more.
Since you have already matched to a hem-onc fellowship, it is probably highly unlikely that you will go un-matched after doing one additional year of research fellowship (assuming you don't need visa sponsorship, which can always be a mess). But whether you will be able to stay in the same program of your research fellowship will depend heavily on the mentor. Things will be better if you have very good connections.
Another downside of your plan would be that a research fellowship will be paid much less than a hem-onc attending, so you will loss one-year of good income