| Depends on your touch. I used to play a five-string down one step with .130-.095-.070-.055-.040 - basically standard/light gauge strings. If you don't mind some fret-clank and/or adjust your technique to a lighter touch, you can try that.
Most probably though, you need to get heavier strings. 145-105-80-60-45-30 will give you a nice uniform tension all across, but still won't be as heavy as a 4-string 40-100-set in standard tuning. For all the tips I've given you, I'd STRONGLY recommend getting D'Addario nickels in singles (Do not get D'Addario prosteels - they are lighter-tensioned than D'Addario nickel roundwounds). What bass do you play? I sure hope you've got a "35-scale. | 
