Your quite right that the original xxx4 shock towers were moulded plastic before the g+ release, but there is no way that the xx4 shock towers will fit a xxx4, i run both cars at club level and they are way different moldings.
The only way the xx4 shock tower will ever fit the xxx4 is if the xxx4 undergoes an X5 tranformation, which uses the xx4 shock towers and other parts( see
www.xfactoryrc.com).
Perhaps you have got hold of some of the original plastic parts instead of the newer graphite version.