It doesn't matter if you did manage to slip that #4 under the lug, if the lug is not rated for that size you may not install it.
Splitting the strands on a conductor is never allowed, (nor is twisting all the bare grounds together and stuffing the result under an empty lug on the bus).
You can find the size wire allowed in the fine print pasted on the side of the panel box, or embossed on the lug itself.
UL Guide Information ZMVV for wire connectors requires "the wire size, wire range or wire combinations" to be "marked on the connector, or on or within the unit container".
