It's a very difficult tax area and you should get advice on the proposed structure.
As a partner, you will be jointly and severally liable for the debts of the business. In other words, all four of you will be fully liable without any limitation on your liability. If one has no assets, the others must pay his share of the liabilities.
The partnership agreement will be the key thing. It must specify the salaries of the partners. It must also show how you can exit if you want to, or if your partners want you out. It should also value the bit the majority partner is selling.
At this stage, the main thing would be to get the accounts to see how profitable the business is. If you sign up to be a partner, there will probably be restrictions on your entering into competition subsequently.
Listen to your boss's proposals and come back with more information, without identifying the business.
Brendan