Beginner Tier has two types of courses--
(1) Fixed (6 courses without any expiry)
(2) Dynamic (2 courses each week with an expiry of 6 days)
Fixed courses have been designed to motivate Newcomers and to lower the barrier to entry.
If you have not completed Achievement Task-4 then you should wait until 28th, from 28th onwards there will be dynamic courses in all three tiers. You can participate in those courses if you meet requirements(eligibility criteria).