Does any breach in contract void the contract all together?

Probably not.  Only a material breach of a contract will excuse the non-breaching party's non-peformance.  When a party breaches a contract in a material way, the non-breaching party generally has two choices.  He/she/it can either rescind the contract, meaning that neither party would have any ongoing obligation, or continue to with the contract  but sue for damages incurred due to the breach.  He/she/it cannot, however, continue under the contract, assert no claim relating to the breach, and use it as an excuse to ignore his/her/its own contractual obligations at some significantly later point in time.  In other words, even assuming that your employer breached (if there is any doubt in the language of the contract, the fact that you went along with the practice for 4 years would support the interpretation that there was no breach) and that the breach was material, having done nothing about the lack of a day off for four years (it is unclear whether you were paid for the extra time you worked, which would make your position even weaker), you can't now use it as an excuse to breach the non-compete. The non-compete may not be unenforceable for other reasons, as such clauses are viewed with some skepticisim by the Courts.  However I don't think the failure to provide a day off will get you off the hook.