Read the brochure about these two. Seems the 4300 was just the model above the 4270.
1975-Marantz-Receiver-Components Pages 34 & 35 compare specs and subsequent pages show each unit with a blurb about it.
Seems the 4300 is the one to pick if you can’t go for the 4400 which gets speedy in fully restored condition.
Oh welcome to ak. Why are you looking at a 4-channel receiver