Edit, Feb 2022: According to user Leigrez, the glitch this guide was written to circumvent, the recruitment menu no longer refreshing, can be fixed by uninstalling and reinstalling the game. This may be faster than following the guide below if you are close to finishing when the glitch occurs.
This guide is for anyone who has encountered the same glitch I have: The recruitment menu freezing/not refreshing with new staff members to choose from.
For this achievement you need to acquire 4 Legendary Staff members (Legendary cards bear an orange mark in the lower-right corner) in 4 separate roles, and add them all to your active crew at the same time. By far the easiest way to find Legendary cards during your career is the recruitment menu under "Crew Management". But for me, this menu always stops replenishing with new potential hires after only a few months and that makes this achievement WAY more of a chore.
To earn this achievement in a career save that has this glitch, the only way is to keep playing until you receive enough Legendaries as random rewards, which could take dozens of hours if you're unlucky like me.
If this is happening to you and you want to get this achievement done swiftly, or it's the only career achievement you have left, I have a strategy to try and speed the process along and compensate for this bug:
1) Start a new career save. Set difficulty to "Easy", damage to "Visual Only", PermaCrash to "Off".
Edit: There is an add-on for this game called the Senior Staff Members Unlock that means you begin your career with 2 legendary staff members already in your crew. Buying this means you only need to find 2 more Legendaries instead of 4. I believe it's about $3. I did this achievement without it but it'll definitely save you a few hours so it's worth mentioning. Credit to VladZelenyak in the comments for this suggestion.
2) Begin your career in WRC2 with a privateer. Hopefully with the difficulty on Easy you will be able to breeze through the Tryouts.
3) Begin your career as normal, by doing training and completing your first event. Feel free to look at the recruitment menu at this stage but it is highly unlikely you'll get a Legendary this early.
4) Put all of your skill points directly into the "Team" branch of R&D, beelining for the "Legendary" skill. This will increase the chances of finding Legendary cards. (If you have skill points to spare already, start unlocking any staff roles you don't yet have access to. You will need these skills in order to hire Staff members in those roles and/or place them into your crew)
5) From this point on you need to be checking the recruitment menu after every single calendar event, and hiring any Legendaries that might appear as soon as you have enough funds.
EDIT: I just discovered another bug you will want to avoid. There is a skill you may have bought that increases the size of your crew reserve from 20 to 35 cards. However the game seems to forget this sometimes, and will delete cards to make room for new ones even when you have plenty of space. I recommend regularly deleting non-legendary cards to keep your total well below 20 staff members if you want to make sure you don't lose any of your Legendaries. I'd say if you go above 15 staff members you're risking it. I just lost a Legendary Financial Director to this glitch today (I had 22/35 cards in my reserve at the time). Learn from my mistake.
6) Continue to progress through the season, completing every rally to make sure you are always rolling that dice for new staff members. At every opportunity, choose Historic Race events to fill out your calendar. This has 2 benefits: 1) You sometimes receive staff members as rewards after these races, just like with championship events. 2) It will earn you quick cash to ensure you always have enough to hire new staff members when a Legendary appears.
7) If you reach the end of a season, try and stay within either WRC2 or WRC2 Pro. DO NOT progress up to WRC, as the rallies in the top category take twice as long and you will start to earn perk cards which will replace staff members in your post-race rewards. Progressing up to WRC will drastically slow the process down, especially if your recruitment menu breaks again.
8) If you join a new team in the off-season, immediately respecialize your R&D tree to acquire the same skills you unlocked before.
9a) Once you have 4 distinct Legendaries, put them into the crew together and your achievement should pop immediately 😄.
9b) If you run into the glitch again before you reach 4 Legendaries (I did when I only had 3), the recruitment menu is unfortunately obsolete from that point onwards. If you are close, keep going, and hopefully you will get the final card(s) you need in short time. If you get the glitch early on in this process, to be honest you may as well delete the save and start again. It just takes too long to get Legendaries only from rewards, even with the right skills unlocked.
Unfortunately I have yet to find a fix to the glitch, hence the need for this guide. I have trawled the internet looking for any mention of this glitch and have come up empty, but this happened to me multiple times across many different career saves so I must assume I'm not the only one who's encountered it.
Hopefully if anyone does have this problem in the future, this guide can provide a quicker alternative to just slogging through many-a-long WRC season waiting for that Orange-tipped card to finally appear on the post-event screen.
(Important note: The recruitment glitch often occurred between play sessions. To give yourself the best chance of success, try to do as much of this guide in one sitting as you can)