$300 is about what they go for, nowadays, and a lot more go for $400 than $300.
Look for things like cracked stock wood, rusted bore, bent or broken sight ears, missing parts.
Look for matching numbers, and do not buy anything marked "DP", which means Drill Purpose. These were shot-out curtain rods that were relegated to cadet groups.
In general, though, the No. 4 was a well-designed and built rifle, with quality metallurgy, and is still in service today.