if i own 40% of a company that is getting an SBA loan, would i be responsible for only 40% of that loan, in case of a default?

That depends on the agreement you enter into with the SBA and the lending bank.  First, since you own 40% of a company, I assume that it is an independent legal entity, most likely a corporation or llc.  If so, you would not in the normal course be personally responsible for any part of its obligations. However, I'm sure that the SBA and lending bank will want personal guarantees from the company's owners.  The extent of your obligation willd epend on the provisions of the guarantees, but most likely you and the other owners will be jointly and severally responsible for the entire loan.  This means that, as between you and the other owners, you would each only be responsible for your pro rata ahsare of the loan, but if for some reason one or more of the owners are unable to meet their obligations, the other owners will have to make up the shortfall.  You could sue the nonpaying owner for reimbursement, but as between you and the SBA and bank,  you would have to pay, and if the other owner had no money with which to reimburse you, you would be stuck.