I'm assuming you mean 4 inch and 6 inch flexwatt? I dont use tubs or racks but the general rule of thumb for belly heat is 1/4 to 1/3 of the cage floor on the heat. The snake needs to have plenty of space to get completely away from the heat if they want.

So to answer your question, if the 6 inch tape covers 1/3 of the cage, then you should be fine. Of course you will want to run a t-stat for any heat source.