lay in 3 fav if 4 fav.... Formula needed

Hi i want lay in a 3 fav horse if the 4 fav its not greater than 2 of the odds od the 3 fav

Example: the 3 fav odds are at 6 and 4 fav the odds are at 9 then DONT BET

               the 3 fav odds are at 6 and 4 fav the odds are at 7 then BET

I suppose a condition checking the odds differens would do the work!

Example: r_4_back_price - r_3_back_price < 2

thanks now work