But why should the increase go down?
Something like this would make more sense I think:
lvl 11: 4000
lvl 12: 8000 (increase by 4000)
lvl 13: 13000 (increase by 5000)
lvl 14: 19000 (increase by 6000)
lvl 15: 26000 (increase by 7000)
lvl 16: 36000 (increase by 8000)
It just feels more... fluent ^^ ofc, if you like it better as it is now, leave it... it's not like it's affecting the gameplay in any bad way.