So you will reach about 1400M hashes/s for 1 5870 - so if you have 2 of them, you have time of one 5970... - and 4870x2 just get about 5-600M hashes/s... - so i think, if you calculate one code with all graphicscards together your max time will be about 2,5 days - 3 days...
It depends on overclocking...
But don't forget - you will have a high electrical bill
- 1 phone in our country with 3 PCs omg 2 days... i think with those graphic cards about 5€