The best replacement for bt4 is 98 custom.

The best alternatives to bt4 are rated as following:
★★★★★  98 custom
★★★  tippmann a5
★★  tippmann 98
★★  bt omega
★  bt3

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Speaking about bt4, one probably shouldn't forget about spoonwep.

Other search terms related to bt4 and their ratings are:
★★★★★ spoonwep
alfa
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bt4
98 custom
tippmann a5