This probably won't influence your decision, but have you thought about:
If your budget if $4,000
Buy a $2,000 build that achieves 80-90% of the performance of a $4,000 build today
18 months from now, spend $600 to upgrade parts
36 months from now, spend $600 to upgrade parts
54 months from now, spend $800 to upgrade parts
Total spent: $4,000, at the end of 4.5 years, you will be at 80-90% of a top tier system
VS
Spend $4,000 now to be in the 95%+ of systems, and at the end of 4.5 years, be at <40% of a top tier system.