12v t3 vs t4 gy6.35 pool light bulb. Whats the difference?

[DenMike99]

I need to the replace the two in my in ground pool light. I believe the socket is the same. So what is the difference? I have the t4 100w available in the area, but will have to order the t3 online. Any reason not to use t4 when they are the same watts and voltage and fit the socket?

 

[blakeusa]

The number after the 'T' is the diameter of the glass in 1/8 inch increments. A T4 bulb, for example, has a diameter of 4/8 or 1/2 inch.