I skid with a D4.
4x4 American, not knowing the year but I'll give you a tip on the best way to appraise used equipment. Go to Machinery Trader or Rock and Dirt. Select auction results. You said it was an older one. That could be an '57 or a '80 like mine. You can narrow it down, though. For auctions on several hundred machines all years, you'll get results like those below.
You Searched For: Manufacturer = CATERPILLAR; Model = D4
Auction Prices Equipment For Sale Prices
Total number of listings found:
690
Highest price found:US $130,000
Lowest price found:US $1,000
Average price
:US $27,596
Average w/out High & Low
: $27,485
Narrow it down for just the years between years 1970 and 1986 it is like this. (You have to select, "High Low Average," after you complete a search.)
You Searched For: Manufacturer = CATERPILLAR; Model = D4; Year From = 1970; Year To = 1986
Auction Prices Equipment For Sale Prices
Total number of listings found:
55
Highest price found:US $70,000
Lowest price found:US $2,500
Average price
:US $11,829 US
Average w/out High & Low
: US $10,907
I won mine playing poker but hope this helps.