What could be the downside of having dedicated tester in Scrum team? The team is composed of 4(developers) + 1(tester).
Your question can be interpreted two ways, but they're both good, so I'll answer both:
What is the downside of isolating testing into one person on a Scrum Team?
Delivering a high-quality product is the whole team's responsibility. Isolating testing into one person is likely to create situations where developers throw work over to testing in the last few days and then try to claim that work is "dev done".
Furthermore, it assumes that a waterfall style "write code and solve quality later in testing" is the best approach to creating a high-quality product. It's become largely accepted in the industry that this is not true and that taking steps to build quality in as the code is written - even going as far as to write your tests first and build to them - is a far superior approach. This is much more difficult with a separate "testing" team member.
What is the downside to having a professional tester join a Scrum team?
None. Great idea. While developers can certainly write tests, professional testers have spent years refining their ability to spot problem areas and risks in applications. This will be a huge benefit to the team. Just remember that they may be asked to help with non-QA tasks and other team members will be a part of the testing process.