By far the best quality 4 inch class APO ( yes I class these as APO,s) is the Skywatcher 100mm ED f9 these are amazing using FPL53 for little money .
1/4 the price of a Tak but 95% of the views , a no-brainer if on a budget like most of us .
Just google Synta 100mm ED f9 reviews there are 1000,s of happy customers world wide.
Brian.