VCF 3.x to 4.x we dont have a path and currenlty its not possible. 

if you want your infra to upgrade to 4.x then you will have to rebuild your VCF 3.x to 4.x , thats the only option right now. 

Since VCF 4.x MGMT has NSx-T and NSX-V is no more there in VCF , we will have to wait for VMware to provide the supported Path