If I understand this properly, if you connected to the vent for the toilet, it no longer is a vent...not good. Now, you say upflush toilet, but then you indicate an ejector pump. To me, this implies a normal toilet that drains to the sump. You can connect the shower to that drain. You need to provide both a trap under the drain for the shower and a vent. Depending on the layout, the toilet's vent might work, but I think not. A toilet normally doesn't require a 4" vent (except maybe where it goes through the roof). Maybe a little more description of what you have is in order.