Only the 27-inch iMacs have user-accessible memory; the memory in the 21-5.inch iMacs are soldered in place, so make sure you customise your iMac to include the amount of memory you'll need for the life of the computer. The 21.5-inch 4K iMac has a very fast processor but from what I can see it's about 20% slower in benchmarks than the 2.5 GHz Intel Core i7 processor in your MacBook Pro.
SSDs make a substantial difference to overall system performance, especially the kind of SSDs Apple puts in the MacBook Pros, so the slower 5400-rpm hard drive in the iMac 4K will be very noticeable. If you've never used an SSD, then you won't know about the difference and it won't bother you, but as you'll be coming from a fast SSD you'll certainly notice it and may get the impression the iMac is slower. I'll give you an example - my Mid 2013 MacBook Air feels faster than the Late 2014 Mac minis which have faster processors because the SSD in the MacBook Airs (and the MacBook Pros) give the impression that the computer is more powerful than it really is. I noticed when I was running lots of apps simultaneously on the MacBook Air it was able to handle it without a problem, but I didn't realise it was paging to disk during heavy workloads as it was running short on memory (has only 4 GB memory), but the PCie-based SSDs are so quick I barely noticed it.
My 27-inch iMac has a 7200-rpm hard drive and I certainly notice how much slower it is compared to my Windows PC that has a SATA-based SSD. Actual system performance is faster than my Windows PC which has an AMD Athlon X4-860K, but app launch times and tasks that involve heavy I/O on the iMac are noticeably slower simply because of the 7200-rpm spinning hard drive. It doesn't personally bother me but it does to others and I can understand that.
You would need to spend at least £1,359 UK / $1,699 US to get a 4K iMac with a 256 GB PCIe-based SSD, so if your MacBook Pro is still serving you fine at the moment but you are considering purchasing an iMac 4K for your desktop set up, then it might be advisable to wait and see if Apple updates their iMacs before the end of the year. A lot of people have their hopes up that the iMacs (and MacBook Pros) will be updated this month, and if they are, it will come as a surprise to everyone if they don't make SSDs a default option in their new iMacs (or at least a Fusion Drive with a minimum 128 GB SSD). From what I've heard, the 1 TB Fusion Drives in the current (Mid 2015) 21.5-inch iMacs only have a 24 GB SSD, which is ridiculous. I don't know if the 1 TB Fusion Drives in the 27-inch iMacs are the same or have a 128 GB SSD.