Rock Island 4000-series Locomotive Numbering Explanation?

I have always wondered why the Rock Island, beginning in the early 1970s, went with 4000-series numbers for new locomotives? I have heard that they picked numbers that would fit in with SP and UP rosters; however, was there any method or reason as to why SD40-2s, U30Cs, GP40-2s, GP38-2s, GP7Ms, and GP9Ms received the numbers that they did? I always found it odd how their locomotive numbers did not venture too far from 1000 prior to the 70s and then all the sudden they began filling out the 4000-series. Also, why did Ingram continue to use the 4000-series for the new GP38-2s and the CRP program when he had his own plan for renumbering, which began with the 3000-series GP40-2 rebuilds in 1979?

If anyone has any insight into this, I would very much appreciate it!

Thanks,
Eric (perplexed)

I don't know for sure, but it makes sense that Rock Island's renumbering into the 4000s was done to fit into Union Pacific's locomotive roster once the merger went through. At the time, Union Pacific didn't have anything numbered in the 4000s. After the merger didn't go through, they probably didn't see any reason to significantly change numbering schemes (and maybe they were hopung that UP would change their mind) so they kept filling in the 4000s. SP didn't have much in the 4000s at the time either (though the 1970s rebuilt SD7/9s ended up there) so if they were hoping for an SP merger it might have made sense to keep putting things in the 4000s. But this is all just speculation.

Thanks for the information, westr! That makes a lot more sense now. I noticed that the numbers SP was using in the 4000s were 4300s, 4400s, and 4800s, so that explains why the Rock went with 4100s, 4700s, and 4900s initially before the CRP and GP38-2 orders. I still wonder why they decided on those specific numbers, or if it was more of a random choice after knowing what SP and UP weren't using.

Here's another thought: I wonder if the Rock had another numbering system in mind for the rebuilt GP40s (like continuing the 4700-series) or if they were always going to use the 3000-series from that program's inception?