Legal Question in Business Law in California

Anything is possible (well, almost anything!) if the shareholders have agreed to it. However, it would be quite unusual for four shareholders to get together and agree that if Shareholder 4 doesn't make some additional investment in the future, that his current holdings will be forfeited back to the corporation. You should hunt for any such provision in various shareholder agreements you may have executed early on; it's remotely possible one of the co-founders may have inserted something of that sort. As to your second question, it is not necessarily a breach of fiduciary duty for a corporation or its officers to fail to repay indebtedness; if the failure is due to personal fault, it's likely also a breach of fiduciary duty, but if the business is failing for reasons not connected with personal fault, failure to repay loans from insiders may be absolutely proper and no breach at all.