Please note I'm not a teacher;
I hope it might help but better wait for an expert opinion.
I just want to try.
If you talk about experience use the present perfect tense.
I've managed four years implementation of a new (I'd put here what kind of system: computer ?) system within a Financial Company of xxxx group.
Ask yourself what is more important in your sentence, ( ie. is it the company or the implementation or time )