Short explanation:
You need 20 000 tier points for gold. The program do not use calendar year and there is no membership year. Tier points do not get reset to zero upon getting blue card. Tier points do expire and gets deleted sequentially from your account after 12 months.
Long explanation:
A3 uses a points-expiry-policy / a rolling status evaluation year. The basic rule is that tier points will expire 12 months after you earned them. This date is labelled "Activity date" in your account and is basically the date of flying. For instance, a retro claimed flight will expire 12 months after flight date, not 12 months after it was credited to your account.
The Internet enrolment bonus is an exception, as it does not expire at all, as long as there is activity in the account.
Your tier account is evaluated every day to see if you meet the requirements for status upgrade. The tier account will first be scanned for points that have expired, delete them and then the remaining tier points will be summed up. If the sum is above 4 000 you will get the blue card upgrade and if it is above 20 000 you will get the gold card upgrade.
Upon getting Blue status, the expiry rules for tier points up until that point will change. My documentation here is based on FT reports, and differ from the apparent interpretation of the T&C. Thus, this section is experimental knowledge, and if you have something that confirms or contradicts this section, please PM me details, ie your account statement .
The 4k miles that earned blue are now secured and will not expire. From this point you need another 16k miles for GOLD. These must be collected in a new 12 month period, starting at the date of your first flight after hitting BLUE. These miles have the same expiry model as before, they will expire 12 months after flight date. If you fail to add 16k miles within 12 months, you will not lose BLUE and you will not gain GOLD. The only thing that will happen is that the oldest miles after getting BLUE status, will expire. So you will have a rolling year to collect 16k miles, and can do so whenever you like.
You will not lose BLUE status, or the first 4k miles it seems, as long as you keep your account active. Check the information for 'keeping card active' under GOLD.
After getting gold status, the collected tier points will no longer expire. The account page will change, and the section labelled "Tier miles soon to expire" will disappear from your account.
To retain status the gold member must now keep his account active. This can be done by using the M&B co-branded credit card or by crediting one (1) tier earning flight to the account within the following 36 months. At this point it is not known if there are any other requirements for re-qualification. Therefore, it is of little use to ask the forum about the re-qualification rules, as we can only provide opinions on this matter!