Mature agile teams with mature agile skill-sets treats development and testing as intertwined activities rather than sequential ones. Development and testing should strive to integrate their activities so as to not be fundamentally separate work streams in one team, or mini waterfalls. If you are unable to do so, you should accept the risks and process deficits associated with the currently implemented workflow.
If you have situations where you have one QA person and four developers, there will be times at the end of the sprint where the developers may sit idle, or pulls in more work into the sprint which may not meet the definition of done at the sprint boundary.
If the above scenario rings true for you then you are not doing Scrum, you are doing waterfall. Activities should be cross functional and leverage a multi faceted team approach to all tasks. During a sprint, developers and testers should be working in tandem. Testers should be involved early and often helping developers design testable features by working on test criteria from the beginning before a single line of code is written, and helping to ensure that tests are written first.
The QA skillsets should be running continuous integration daily, so a tight and immediate feedback loop exists between testing and development. By working with developers rather than being treated as a separate follow-on activity, QA becomes an intrinsic part of the design and development cycles rather than an externality.