What is on the old server that needs to be moved to new one? Any hosted programs? Or just file storage? Only 4 computers...probably would be starting from scratch, just copying folders/files over. Barely takes 15 minutes per workstation..secure local Admin account, unjoin, bounce, join new, bounce...copy cherry picked folders of old profile back.
Migration..you have at least several hours just cleaning up old group policies and restructuring ADUC and defaults...not to mention often little things (remnants) still linger in active directory that can pester you down the road.
Migrating from vanilla server to newer vanilla server is quite easy. But migrating from a special "suite" flavor of server like SBS or it's half breed Essentials...there is a TON of unique things in active directory to clean up. Lots of techs just ignore it...but that's a poor job and always leads to quirks and failing audit tests and lots of odd behavior down the road on client rigs. Really have to spend quite a few hours rolling up your sleeves and cleaning "unique" things from the prior SBS/Essentials servers